<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision: 297028 $ -->
 <chapter xml:id="language.functions" xmlns="http://docbook.org/ns/docbook">
  <title>Functions</title>

  <sect1 xml:id="functions.user-defined">
   <title>User-defined functions</title>
 
   <para>
    A function may be defined using syntax such as the following:
   </para>
   <para>
    <example>
     <title>Pseudo code to demonstrate function uses</title>
     <programlisting role="php">
<![CDATA[
<?php
function foo($arg_1, $arg_2, /* ..., */ $arg_n)
{
    echo "Example function.\n";
    return $retval;
}
?>
]]>
     </programlisting>
    </example>
   </para>
   
   <simpara>
    Any valid PHP code may appear inside a function, even other
    functions and <link linkend="keyword.class">class</link>
    definitions.
   </simpara>
   <para>
    Function names follow the same rules as other labels in PHP. A
    valid function name starts with a letter or underscore, followed
    by any number of letters, numbers, or underscores. As a regular
    expression, it would be expressed thus:
    <literal>[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*</literal>.
   </para>
   &tip.userlandnaming;
   <simpara>
    Functions need not be defined before they are referenced,
    <emphasis>except</emphasis> when a function is conditionally defined as
    shown in the two examples below.
   </simpara>
   <para>
    When a function is defined in a conditional manner such as the two
    examples shown. Its definition must be processed <emphasis>prior</emphasis>
    to being called.
   </para>
   <para>
    <example>
     <title>Conditional functions</title>
     <programlisting role="php">
<![CDATA[
<?php

$makefoo = true;

/* We can't call foo() from here 
   since it doesn't exist yet,
   but we can call bar() */

bar();

if ($makefoo) {
  function foo()
  {
    echo "I don't exist until program execution reaches me.\n";
  }
}

/* Now we can safely call foo()
   since $makefoo evaluated to true */

if ($makefoo) foo();

function bar() 
{
  echo "I exist immediately upon program start.\n";
}

?>
]]>
     </programlisting>
    </example>
   </para>
   <para>
    <example>
     <title>Functions within functions</title>
     <programlisting role="php">
<![CDATA[
<?php
function foo() 
{
  function bar() 
  {
    echo "I don't exist until foo() is called.\n";
  }
}

/* We can't call bar() yet
   since it doesn't exist. */

foo();

/* Now we can call bar(),
   foo()'s processesing has
   made it accessible. */

bar();

?>  
]]>
     </programlisting>
    </example>
   </para>
   <para>
    All functions and classes in PHP have the global scope - they can be
    called outside a function even if they were defined inside and vice versa.
   </para>
   <simpara>
    PHP does not support function overloading, nor is it possible to
    undefine or redefine previously-declared functions.
   </simpara>
   <note>
    <simpara>
     Function names are case-insensitive, though it is usually good form
     to call functions as they appear in their declaration.
    </simpara>
   </note>   
   <simpara>
    Both <link linkend="functions.variable-arg-list">variable number of
    arguments</link> and <link linkend="functions.arguments.default">default
    arguments</link> are supported in functions. See also the function
    references for
    <function>func_num_args</function>,
    <function>func_get_arg</function>, and
    <function>func_get_args</function> for more information.
   </simpara>
   
   <para>
    It is possible to call recursive functions in PHP. However avoid recursive
    function/method calls with over 100-200 recursion levels as it can smash
    the stack and cause a termination of the current script.
    <example>
     <title>Recursive functions</title>
     <programlisting role="php">
<![CDATA[
<?php
function recursion($a)
{
    if ($a < 20) {
        echo "$a\n";
        recursion($a + 1);
    }
}
?>
]]>
     </programlisting>
    </example>
   </para>

  </sect1>
 
  <sect1 xml:id="functions.arguments">
   <title>Function arguments</title>
 
   <simpara>
    Information may be passed to functions via the argument list,
    which is a comma-delimited list of expressions.
   </simpara> 
   <para>
    PHP supports passing arguments by value (the default), <link
    linkend="functions.arguments.by-reference">passing by
    reference</link>, and <link
    linkend="functions.arguments.default">default argument
    values</link>. <link linkend="functions.variable-arg-list">Variable-length
    argument lists</link> are also supported, see also the function references for
    <function>func_num_args</function>,
    <function>func_get_arg</function>, and
    <function>func_get_args</function> for more information.
   </para>
   <para>
    <example>
     <title>Passing arrays to functions</title>
     <programlisting role="php">
<![CDATA[
<?php
function takes_array($input)
{
    echo "$input[0] + $input[1] = ", $input[0]+$input[1];
}
?>
]]>
     </programlisting>
    </example>
   </para>
 
   <sect2 xml:id="functions.arguments.by-reference">
    <title>Making arguments be passed by reference</title>
 
    <simpara>
     By default, function arguments are passed by value (so that if
     the value of the argument within the function is changed, it does
     not get changed outside of the function). To allow a function to modify its
     arguments, they must be passed by reference.
    </simpara>
    <para>
     To have an argument to a function always passed by reference, prepend an
     ampersand (&amp;) to the argument name in the function definition:
    </para>
    <para>
     <example>
      <title>Passing function parameters by reference</title>
      <programlisting role="php">
<![CDATA[
<?php
function add_some_extra(&$string)
{
    $string .= 'and something extra.';
}
$str = 'This is a string, ';
add_some_extra($str);
echo $str;    // outputs 'This is a string, and something extra.'
?>
]]>
      </programlisting>
     </example>
    </para>
 
   </sect2>
 
   <sect2 xml:id="functions.arguments.default">
    <title>Default argument values</title>
 
    <para>
     A function may define C++-style default values for scalar
     arguments as follows:
    </para>
    <para>
     <example>
      <title>Use of default parameters in functions</title>
      <programlisting role="php">
<![CDATA[
<?php
function makecoffee($type = "cappuccino")
{
    return "Making a cup of $type.\n";
}
echo makecoffee();
echo makecoffee(null);
echo makecoffee("espresso");
?>
]]>
      </programlisting>
      &example.outputs;
      <screen>
<![CDATA[
Making a cup of cappuccino.
Making a cup of .
Making a cup of espresso.
]]>
      </screen>
     </example>
    </para>
    <para>
     PHP also allows the use of <type>array</type>s and the special type &null;
     as default values, for example:
    </para>
    <para>
     <example>
      <title>Using non-scalar types as default values</title>
      <programlisting role="php">
<![CDATA[
<?php
function makecoffee($types = array("cappuccino"), $coffeeMaker = NULL)
{
    $device = is_null($coffeeMaker) ? "hands" : $coffeeMaker;
    return "Making a cup of ".join(", ", $types)." with $device.\n";
}
echo makecoffee();
echo makecoffee(array("cappuccino", "lavazza"), "teapot");
?>
]]>
      </programlisting>
     </example>
    
    </para>
    <simpara>
     The default value must be a constant expression, not (for
     example) a variable, a class member or a function call.
    </simpara>
    <para>
     Note that when using default arguments, any defaults should be on
     the right side of any non-default arguments; otherwise, things
     will not work as expected. Consider the following code snippet:
    </para>
    <para>
     <example>
      <title>Incorrect usage of default function arguments</title>
      <programlisting role="php">
<![CDATA[
<?php
function makeyogurt($type = "acidophilus", $flavour)
{
    return "Making a bowl of $type $flavour.\n";
}
 
echo makeyogurt("raspberry");   // won't work as expected
?>
]]>
      </programlisting>
      &example.outputs;
      <screen>
<![CDATA[
Warning: Missing argument 2 in call to makeyogurt() in 
/usr/local/etc/httpd/htdocs/phptest/functest.html on line 41
Making a bowl of raspberry .
]]>
      </screen>
     </example>
    </para>
    <para>
     Now, compare the above with this:
    </para>
    <para>
     <example>
      <title>Correct usage of default function arguments</title>
      <programlisting role="php">
<![CDATA[
<?php
function makeyogurt($flavour, $type = "acidophilus")
{
    return "Making a bowl of $type $flavour.\n";
}
 
echo makeyogurt("raspberry");   // works as expected
?>
]]>
      </programlisting>
      &example.outputs;
      <screen>
<![CDATA[
Making a bowl of acidophilus raspberry.
]]>
      </screen>
     </example>
    </para>
    <note>
     <simpara>
      As of PHP 5, default values may be passed by reference.
     </simpara>
    </note>

   </sect2>

   <sect2 xml:id="functions.variable-arg-list">
    <title>Variable-length argument lists</title>

    <simpara>
     PHP 4 and above has support for variable-length argument lists in
     user-defined functions. This is really quite easy, using the
     <function>func_num_args</function>,
     <function>func_get_arg</function>, and
     <function>func_get_args</function> functions.
    </simpara>

    <simpara>
     No special syntax is required, and argument lists may still be
     explicitly provided with function definitions and will behave as
     normal.
    </simpara>

   </sect2>

  </sect1>
 
  <sect1 xml:id="functions.returning-values">
   <title>Returning values</title>
 
   <para>
    Values are returned by using the optional return statement. Any
    type may be returned, including arrays and objects. This causes the
    function to end its execution immediately and pass control back to
    the line from which it was called. See <function>return</function>
    for more information.
   </para>
   <note>
    <para>
     If the <function>return</function> is omitted the value &null; will be
     returned.
    </para>
   </note>
   <para>
    <example>
     <title>Use of <function>return</function></title>
     <programlisting role="php">
<![CDATA[
<?php
function square($num)
{
    return $num * $num;
}
echo square(4);   // outputs '16'.
?>
]]>
     </programlisting>
    </example>
   </para>
      
   <para>
    A function can not return multiple values, but similar results can be
    obtained by returning an array.
   </para>
   <para>
    <example>
     <title>Returning an array to get multiple values</title>
     <programlisting role="php">
<![CDATA[
<?php
function small_numbers()
{
    return array (0, 1, 2);
}
list ($zero, $one, $two) = small_numbers();
?>
]]>
     </programlisting>
    </example>
   </para>
   <para>
    To return a reference from a function, use the reference operator &amp; in
    both the function declaration and when assigning the returned value to a
    variable:
   </para>
   <para>
    <example>
     <title>Returning a reference from a function</title>
     <programlisting role="php">
<![CDATA[
<?php
function &returns_reference()
{
    return $someref;
}

$newref =& returns_reference();
?>
]]>
     </programlisting>
    </example>
   </para>
   <simpara>
    For more information on references, please check out <link
    linkend="language.references">References Explained</link>.
   </simpara>
  </sect1>
 
  <sect1 xml:id="functions.variable-functions">
   <title>Variable functions</title>

   <para>
    PHP supports the concept of variable functions. This means that if
    a variable name has parentheses appended to it, PHP will look for
    a function with the same name as whatever the variable evaluates
    to, and will attempt to execute it. Among other things, this can
    be used to implement callbacks, function tables, and so forth.
   </para>
   <para>
    Variable functions won't work with language constructs such 
    as <function>echo</function>, <function>print</function>,
    <function>unset</function>, <function>isset</function>,
    <function>empty</function>, <function>include</function>,
    <function>require</function> and the like. Utilize wrapper functions to make
    use of any of these constructs as variable functions.
   </para>
   <para>
    <example>
     <title>Variable function example</title>
     <programlisting role="php">
<![CDATA[
<?php
function foo() {
    echo "In foo()<br />\n";
}

function bar($arg = '')
{
    echo "In bar(); argument was '$arg'.<br />\n";
}

// This is a wrapper function around echo
function echoit($string)
{
    echo $string;
}

$func = 'foo';
$func();        // This calls foo()

$func = 'bar';
$func('test');  // This calls bar()

$func = 'echoit';
$func('test');  // This calls echoit()
?>
]]>
     </programlisting>
    </example>
   </para>
   <para>
    An object method can also be called with the variable functions syntax.
    <example>
     <title>Variable method example</title>
     <programlisting role="php">
<![CDATA[
<?php
class Foo
{
    function Variable()
    {
        $name = 'Bar';
        $this->$name(); // This calls the Bar() method
    }
    
    function Bar()
    {
        echo "This is Bar";
    }
}

$foo = new Foo();
$funcname = "Variable";
$foo->$funcname();  // This calls $foo->Variable()

?>
]]>
     </programlisting>
    </example>
   </para>
   <para>
    See also <function>call_user_func</function>,
    <link linkend="language.variables.variable">
    variable variables</link> and <function>function_exists</function>.
   </para>
  </sect1>
  
  <sect1 xml:id="functions.internal">
   <title>Internal (built-in) functions</title>
   
   <para>
    PHP comes standard with many functions and constructs. There are also
    functions that require specific PHP extensions compiled in, otherwise 
    fatal "undefined function" errors will appear. For example, to use 
    <link linkend="ref.image">image</link> functions such as 
    <function>imagecreatetruecolor</function>, PHP must be compiled with
    <productname>GD</productname> support. Or, to use
    <function>mysql_connect</function>, PHP must be compiled with
    <link linkend="ref.mysql">MySQL</link> support. There are many core functions
    that are included in every version of PHP, such as the
    <link linkend="ref.strings">string</link> and 
    <link linkend="ref.var">variable</link> functions. A call 
    to <function>phpinfo</function> or
    <function>get_loaded_extensions</function> will show which extensions are
    loaded into PHP. Also note that many extensions are enabled by default and
    that the PHP manual is split up by extension. See the
    <link linkend="configuration">configuration</link>,
    <link linkend="install">installation</link>, and individual
    extension chapters, for information on how to set up PHP.
   </para>
   <para>
    Reading and understanding a function's prototype is explained within the
    manual section titled <link linkend="about.prototypes">how to read a
    function definition</link>. It's important to realize what a function
    returns or if a function works directly on a passed in value. For example,
    <function>str_replace</function> will return the modified string while 
    <function>usort</function> works on the actual passed in variable
    itself. Each manual page also has specific information for each
    function like information on function parameters, behavior changes,
    return values for both success and failure, and availability information.
    Knowing these important (yet often subtle) differences is crucial for
    writing correct PHP code.
   </para>
   <note>
    <simpara>
     If the parameters given to a function are not what it expects, such as 
     passing an <type>array</type> where a <type>string</type> is expected, 
     the return value of the function is undefined. In this case it will
     likely return &null; but this is just a convention, and cannot be relied 
     upon.
    </simpara>
   </note>
   <para>
    See also <function>function_exists</function>, 
    <link linkend="funcref">the function reference</link>,
    <function>get_extension_funcs</function>, and 
    <function>dl</function>.
   </para>
  </sect1>
  
  <sect1 xml:id="functions.anonymous">
   <title>Anonymous functions</title>

   <simpara>
    Anonymous functions, also known as <literal>closures</literal>, allow the
    creation of functions which have no specified name. They are most useful as
    the value of <link linkend="language.types.callback">callback</link>
    parameters, but they have many other uses.
   </simpara>

   <example>
    <title>Anonymous function example</title>
    <programlisting role="php">
<![CDATA[
<?php
echo preg_replace_callback('~-([a-z])~', function ($match) {
    return strtoupper($match[1]);
}, 'hello-world');
// outputs helloWorld
?>
]]>
    </programlisting>
   </example>

   <simpara>
    Closures can also be used as the values of variables; PHP automatically 
    converts such expressions into instances of the
    <classname>Closure</classname> internal class. Assigning a closure to a
    variable uses the same syntax as any other assignment, including the
    trailing semicolon:
   </simpara>

   <example>
    <title>Anonymous function variable assignment example</title>
    <programlisting role="php">
<![CDATA[
<?php
$greet = function($name)
{
    printf("Hello %s\r\n", $name);
};

$greet('World');
$greet('PHP');
?>
]]>
    </programlisting>
   </example>
   
   <simpara>
    Closures may also inherit variables from the parent scope. Any such
    variables must be declared in the function header. Inheriting variables from
    the parent scope is <emphasis>not</emphasis> the same as using global
    variables. Global variables exist in the global scope, which is the same no
    matter what function is executing. The parent scope of a closure is the
    function in which the closure was declared (not necessarily the function it
    was called from). See the following example:
   </simpara>

   <example>
    <title>Closures and scoping</title>
    <programlisting role="php">
<![CDATA[
<?php
// A basic shopping cart which contains a list of added products
// and the quantity of each product. Includes a method which
// calculates the total price of the items in the cart using a
// closure as a callback.
class Cart
{
    const PRICE_BUTTER  = 1.00;
    const PRICE_MILK    = 3.00;
    const PRICE_EGGS    = 6.95;

    protected   $products = array();
    
    public function add($product, $quantity)
    {
        $this->products[$product] = $quantity;
    }
    
    public function getQuantity($product)
    {
        return isset($this->products[$product]) ? $this->products[$product] :
               FALSE;
    }
    
    public function getTotal($tax)
    {
        $total = 0.00;
        
        $callback =
            function ($quantity, $product) use ($tax, &$total)
            {
                $pricePerItem = constant(__CLASS__ . "::PRICE_" .
                    strtoupper($product));
                $total += ($pricePerItem * $quantity) * ($tax + 1.0);
            };
        
        array_walk($this->products, $callback);
        return round($total, 2);
    }
}

$my_cart = new Cart;

// Add some items to the cart
$my_cart->add('butter', 1);
$my_cart->add('milk', 3);
$my_cart->add('eggs', 6);

// Print the total with a 5% sales tax.
print $my_cart->getTotal(0.05) . "\n";
// The result is 54.29
?>
]]>
    </programlisting>
   </example>
   
   <simpara>
    Anonymous functions are currently implemented using the
    <link linkend="reserved.classes.closure">
    <classname>Closure</classname></link> class. This is an implementation
    detail and should not be relied upon.
   </simpara>
   
   <note>
    <simpara>
     Anonymous functions are available since PHP 5.3.0.
    </simpara>
   </note>

   <note>
    <simpara>
     It is possible to use <function>func_num_args</function>,
     <function>func_get_arg</function>, and <function>func_get_args</function>
     from within a closure.
    </simpara>
   </note>

  </sect1>

 </chapter>
 
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