<?xml version="1.0" encoding="utf-8"?>
<sect1 xml:id="language.operators.comparison">
 <title>Comparison Operators</title>
 <titleabbrev>Comparison</titleabbrev>
 <simpara>
  Comparison operators, as their name implies, allow you to compare
  two values.  You may also be interested in viewing
  <link linkend="types.comparisons">the type comparison tables</link>,
  as they show examples of various type related comparisons.
 </simpara>
 <table>
  <title>Comparison Operators</title>
  <tgroup cols="3">
   <thead>
    <row>
     <entry>Example</entry>
     <entry>Name</entry>
     <entry>Result</entry>
    </row>
   </thead>
   <tbody>
    <row>
     <entry>$a == $b</entry>
     <entry>Equal</entry>
     <entry>&true; if <varname>$a</varname> is equal to <varname>$b</varname> after type juggling.</entry>
    </row>
    <row>
     <entry>$a === $b</entry>
     <entry>Identical</entry>
     <entry>
      &true; if <varname>$a</varname> is equal to <varname>$b</varname>, and they are of the same
      type.
     </entry>
    </row>
    <row>
     <entry>$a != $b</entry>
     <entry>Not equal</entry>
     <entry>&true; if <varname>$a</varname> is not equal to <varname>$b</varname> after type juggling.</entry>
    </row>
    <row>
     <entry>$a &lt;&gt; $b</entry>
     <entry>Not equal</entry>
     <entry>&true; if <varname>$a</varname> is not equal to <varname>$b</varname> after type juggling.</entry>
    </row>
    <row>
     <entry>$a !== $b</entry>
     <entry>Not identical</entry>
     <entry>
      &true; if <varname>$a</varname> is not equal to <varname>$b</varname>, or they are not of the same
      type.
     </entry>
    </row>
    <row>
     <entry>$a &lt; $b</entry>
     <entry>Less than</entry>
     <entry>&true; if <varname>$a</varname> is strictly less than <varname>$b</varname>.</entry>
    </row>
    <row>
     <entry>$a &gt; $b</entry>
     <entry>Greater than</entry>
     <entry>&true; if <varname>$a</varname> is strictly greater than <varname>$b</varname>.</entry>
    </row>
    <row>
     <entry>$a &lt;= $b</entry>
     <entry>Less than or equal to</entry>
     <entry>&true; if <varname>$a</varname> is less than or equal to <varname>$b</varname>.</entry>
    </row>
    <row>
     <entry>$a &gt;= $b</entry>
     <entry>Greater than or equal to</entry>
     <entry>&true; if <varname>$a</varname> is greater than or equal to <varname>$b</varname>.</entry>
    </row>
    <row>
     <entry>$a &lt;=&gt; $b</entry>
     <entry>Spaceship</entry>
     <entry>
      An <type>int</type> less than, equal to, or greater than zero when
      <varname>$a</varname> is less than, equal to, or greater than
      <varname>$b</varname>, respectively.
     </entry>
    </row>
   </tbody>
  </tgroup>
 </table>
 <para>
  If both operands are
  <link linkend="language.types.numeric-strings">numeric strings</link>,
  or one operand is a number and the other one is a
  <link linkend="language.types.numeric-strings">numeric string</link>,
  then the comparison is done numerically.
  These rules also apply to the
  <link linkend="control-structures.switch">switch</link> statement.
  The type conversion does not take place when the comparison is
  <literal>===</literal> or <literal>!==</literal> as this involves
  comparing the type as well as the value.
 </para>

 <warning>
  <para>
   Prior to PHP 8.0.0, if a <type>string</type> is compared to a number
   or a numeric string then the <type>string</type> was converted to a
   number before performing the comparison. This can lead to surprising
   results as can be seen with the following example:
   <informalexample>
    <programlisting role="php">
<![CDATA[
<?php
var_dump(0 == "a");
var_dump("1" == "01");
var_dump("10" == "1e1");
var_dump(100 == "1e2");

switch ("a") {
case 0:
    echo "0";
    break;
case "a":
    echo "a";
    break;
}
?>
]]>
    </programlisting>
    &example.outputs.7;
    <screen>
<![CDATA[
bool(true)
bool(true)
bool(true)
bool(true)
0
]]>
    </screen>
    &example.outputs.8;
    <screen>
<![CDATA[
bool(false)
bool(true)
bool(true)
bool(true)
a
]]>
    </screen>
   </informalexample>
  </para>
 </warning>

 <para>
  <informalexample>
   <programlisting role="php">
<![CDATA[
<?php
// Integers
echo 1 <=> 1; // 0
echo 1 <=> 2; // -1
echo 2 <=> 1; // 1

// Floats
echo 1.5 <=> 1.5; // 0
echo 1.5 <=> 2.5; // -1
echo 2.5 <=> 1.5; // 1

// Strings
echo "a" <=> "a"; // 0
echo "a" <=> "b"; // -1
echo "b" <=> "a"; // 1

echo "a" <=> "aa"; // -1
echo "zz" <=> "aa"; // 1

// Arrays
echo [] <=> []; // 0
echo [1, 2, 3] <=> [1, 2, 3]; // 0
echo [1, 2, 3] <=> []; // 1
echo [1, 2, 3] <=> [1, 2, 1]; // 1
echo [1, 2, 3] <=> [1, 2, 4]; // -1

// Objects
$a = (object) ["a" => "b"];
$b = (object) ["a" => "b"];
echo $a <=> $b; // 0

$a = (object) ["a" => "b"];
$b = (object) ["a" => "c"];
echo $a <=> $b; // -1

$a = (object) ["a" => "c"];
$b = (object) ["a" => "b"];
echo $a <=> $b; // 1

// not only values are compared; keys must match
$a = (object) ["a" => "b"];
$b = (object) ["b" => "b"];
echo $a <=> $b; // 1

?>
]]>

   </programlisting>
  </informalexample>
 </para>

 <para>
  For various types, comparison is done according to the following
  table (in order).
 </para>
 <table xml:id="language.operators.comparison.types">
  <title>Comparison with Various Types</title>
  <tgroup cols="3">
   <thead>
    <row>
     <entry>Type of Operand 1</entry>
     <entry>Type of Operand 2</entry>
     <entry>Result</entry>
    </row>
   </thead>
   <tbody>
    <row>
     <entry><type>null</type> or <type>string</type></entry>
     <entry><type>string</type></entry>
     <entry>Convert &null; to "", numerical or lexical comparison</entry>
    </row>
    <row>
     <entry><type>bool</type> or <type>null</type></entry>
     <entry>anything</entry>
     <entry>Convert both sides to <type>bool</type>, &false; &lt; &true;</entry>
    </row>
    <row>
     <entry><type>object</type></entry>
     <entry><type>object</type></entry>
     <entry>Built-in classes can define its own comparison, different classes
      are incomparable, same class see <link
       linkend="language.oop5.object-comparison">Object Comparison</link>
     </entry>
    </row>
    <row>
     <entry><type>string</type>, <type>resource</type>, <type>int</type> or <type>float</type></entry>
     <entry><type>string</type>, <type>resource</type>, <type>int</type> or <type>float</type></entry>
     <entry>Translate strings and resources to numbers, usual math</entry>
    </row>
    <row>
     <entry><type>array</type></entry>
     <entry><type>array</type></entry>
     <entry>Array with fewer members is smaller, if key from operand 1 is not
      found in operand 2 then arrays are incomparable, otherwise - compare
      value by value (see following example)</entry>
    </row>
    <row>
     <entry><type>object</type></entry>
     <entry>anything</entry>
     <entry><type>object</type> is always greater</entry>
    </row>
    <row>
     <entry><type>array</type></entry>
     <entry>anything</entry>
     <entry><type>array</type> is always greater</entry>
    </row>
   </tbody>
  </tgroup>
 </table>

 <para>
  <example>
   <title>Boolean/null comparison</title>
   <programlisting role="php">
<![CDATA[
<?php
// Bool and null are compared as bool always
var_dump(1 == TRUE);  // TRUE - same as (bool) 1 == TRUE
var_dump(0 == FALSE); // TRUE - same as (bool) 0 == FALSE
var_dump(100 < TRUE); // FALSE - same as (bool) 100 < TRUE
var_dump(-10 < FALSE);// FALSE - same as (bool) -10 < FALSE
var_dump(min(-100, -10, NULL, 10, 100)); // NULL - (bool) NULL < (bool) -100 is FALSE < TRUE
?>
]]>
   </programlisting>
  </example>
 </para>

 <para>
  <example>
   <title>Transcription of standard array comparison</title>
   <programlisting role="php">
<![CDATA[
<?php
// Arrays are compared like this with standard comparison operators as well as the spaceship operator.
function standard_array_compare($op1, $op2)
{
    if (count($op1) < count($op2)) {
        return -1; // $op1 < $op2
    } elseif (count($op1) > count($op2)) {
        return 1; // $op1 > $op2
    }
    foreach ($op1 as $key => $val) {
        if (!array_key_exists($key, $op2)) {
            return 1;
        } elseif ($val < $op2[$key]) {
            return -1;
        } elseif ($val > $op2[$key]) {
            return 1;
        }
    }
    return 0; // $op1 == $op2
}
?>
]]>
   </programlisting>
  </example>
 </para>

 <warning>
  <title>Comparison of floating point numbers</title>

  <para>
   Because of the way <type>float</type>s are represented internally, you
   should not test two <type>float</type>s for equality.
  </para>

  <para>
   See the documentation for <type>float</type> for more information.
  </para>
 </warning>

 <note>
  <simpara>
   Be aware that PHP's type juggling is not always obvious when comparing values of different types,
   particularly comparing &integer;s to &boolean;s or &integer;s to &string;s. It is therefore generally
   advisable to use <literal>===</literal> and <literal>!==</literal> comparisons rather than
   <literal>==</literal> and <literal>!=</literal> in most cases.
  </simpara>
 </note>

 <sect2 xml:id="language.operators.comparison.incomparable">
  <title>Incomparable Values</title>
  <simpara>
   While identity comparison (<literal>===</literal> and <literal>!==</literal>)
   can be applied to arbitrary values, the other comparison operators should only be
   applied to comparable values. The result of comparing incomparable values is
   undefined, and should not be relied upon.
  </simpara>
 </sect2>

 <sect2 role="seealso">
  &reftitle.seealso;
  <para>
   <simplelist>
    <member><function>strcasecmp</function></member>
    <member><function>strcmp</function></member>
    <member><link linkend="language.operators.array">Array operators</link></member>
    <member><link linkend="language.types">Types</link></member>
   </simplelist>
  </para>
 </sect2>

 <sect2 xml:id="language.operators.comparison.ternary">
  <title>Ternary Operator</title>
  <para>
   Another conditional operator is the "?:" (or ternary) operator.
   <example>
    <title>Assigning a default value</title>
    <programlisting role="php">
<![CDATA[
<?php
// Example usage for: Ternary Operator
$action = (empty($_POST['action'])) ? 'default' : $_POST['action'];

// The above is identical to this if/else statement
if (empty($_POST['action'])) {
    $action = 'default';
} else {
    $action = $_POST['action'];
}
?>
]]>
    </programlisting>
   </example>
   The expression <literal>(expr1) ? (expr2) : (expr3)</literal>
   evaluates to <replaceable>expr2</replaceable> if
   <replaceable>expr1</replaceable> evaluates to &true;, and
   <replaceable>expr3</replaceable> if
   <replaceable>expr1</replaceable> evaluates to &false;.
  </para>
  <para>
   It is possible to leave out the middle part of the ternary operator.
   Expression <literal>expr1 ?: expr3</literal> evaluates to
   the result of <replaceable>expr1</replaceable> if <replaceable>expr1</replaceable>
   evaluates to &true;, and <replaceable>expr3</replaceable> otherwise.
   <replaceable>expr1</replaceable> is only evaluated once in this case.
  </para>
  <note>
   <simpara>
    Please note that the ternary operator is an expression, and that it
    doesn't evaluate to a variable, but to the result of an expression. This
    is important to know if you want to return a variable by reference.
    The statement <literal>return $var == 42 ? $a : $b;</literal> in a
    return-by-reference function will therefore not work and a warning is
    issued.
   </simpara>
  </note>
  <note>
   <para>
    It is recommended to avoid "stacking" ternary expressions.
    PHP's behaviour when using more than one unparenthesized ternary operator within a single
    expression is non-obvious compared to other languages.
    Indeed prior to PHP 8.0.0, ternary expressions were evaluated left-associative,
    instead of right-associative like most other programming languages.
    Relying on left-associativity is deprecated as of PHP 7.4.0.
    As of PHP 8.0.0, the ternary operator is non-associative.
    <example>
     <title>Non-obvious Ternary Behaviour</title>
     <programlisting role="php">
<![CDATA[
<?php
// on first glance, the following appears to output 'true'
echo (true ? 'true' : false ? 't' : 'f');

// however, the actual output of the above is 't' prior to PHP 8.0.0
// this is because ternary expressions are left-associative

// the following is a more obvious version of the same code as above
echo ((true ? 'true' : false) ? 't' : 'f');

// here, one can see that the first expression is evaluated to 'true', which
// in turn evaluates to (bool) true, thus returning the true branch of the
// second ternary expression.
?>
]]>
     </programlisting>
    </example>
   </para>
  </note>
  <note>
   <para>
    Chaining of short-ternaries (<literal>?:</literal>), however, is stable and behaves reasonably.
    It will evaluate to the first argument that evaluates to a non-falsy value.  Note that undefined
    values will still raise a warning.
    <example>
     <title>Short-ternary chaining</title>
     <programlisting role="php">
<![CDATA[
<?php
echo 0 ?: 1 ?: 2 ?: 3, PHP_EOL; //1
echo 0 ?: 0 ?: 2 ?: 3, PHP_EOL; //2
echo 0 ?: 0 ?: 0 ?: 3, PHP_EOL; //3
?>
]]>
     </programlisting>
    </example>
   </para>
  </note>
 </sect2>

 <sect2 xml:id="language.operators.comparison.coalesce">
  <title>Null Coalescing Operator</title>
  <para>
   Another useful shorthand operator is the "??" (or null coalescing) operator.
   <example>
    <title>Assigning a default value</title>
    <programlisting role="php">
<![CDATA[
<?php
// Example usage for: Null Coalesce Operator
$action = $_POST['action'] ?? 'default';

// The above is identical to this if/else statement
if (isset($_POST['action'])) {
    $action = $_POST['action'];
} else {
    $action = 'default';
}
?>
]]>
    </programlisting>
   </example>
   The expression <literal>(expr1) ?? (expr2)</literal> evaluates to
   <replaceable>expr2</replaceable> if <replaceable>expr1</replaceable> is
   &null;, and <replaceable>expr1</replaceable> otherwise.
  </para>
  <para>
   In particular, this operator does not emit a notice or warning if the left-hand side
   value does not exist, just like <function>isset</function>. This is especially
   useful on array keys.
  </para>
  <note>
   <simpara>
    Please note that the null coalescing operator is an expression, and that it
    doesn't evaluate to a variable, but to the result of an expression. This
    is important to know if you want to return a variable by reference.
    The statement <literal>return $foo ?? $bar;</literal> in a
    return-by-reference function will therefore not work and a warning is
    issued.
   </simpara>
  </note>
  <note>
   <para>
    The null coalescing operator has low precedence. That means if mixing it
    with other operators (such as string concatenation or arithmetic operators)
    parentheses will likely be required.
   </para>
   <programlisting role="php">
<![CDATA[
<?php
// Raises a warning that $name is undefined.
print 'Mr. ' . $name ?? 'Anonymous';

// Prints "Mr. Anonymous"
print 'Mr. ' . ($name ?? 'Anonymous');
?>
]]>
   </programlisting>
  </note>
  <note>
   <para>
    Please note that the null coalescing operator allows for simple nesting:
    <example>
     <title>Nesting null coalescing operator</title>
     <programlisting role="php">
<![CDATA[
<?php

$foo = null;
$bar = null;
$baz = 1;
$qux = 2;

echo $foo ?? $bar ?? $baz ?? $qux; // outputs 1

?>
]]>
     </programlisting>
    </example>
   </para>
  </note>
 </sect2>
</sect1>