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<?xml version="1.0" encoding="utf-8"?>
<sect1 xml:id="language.operators.increment">
<title>Incrementing/Decrementing Operators</title>
<titleabbrev>Increment and Decrement</titleabbrev>
<para>
PHP supports pre- and post-increment and decrement operators.
Those unary operators allow to increment or decrement the value by one.
</para>
<table>
<title>Increment/decrement Operators</title>
<tgroup cols="3">
<thead>
<row>
<entry>Example</entry>
<entry>Name</entry>
<entry>Effect</entry>
</row>
</thead>
<tbody>
<row>
<entry>++$a</entry>
<entry>Pre-increment</entry>
<entry>Increments <varname>$a</varname> by one, then returns <varname>$a</varname>.</entry>
</row>
<row>
<entry>$a++</entry>
<entry>Post-increment</entry>
<entry>Returns <varname>$a</varname>, then increments <varname>$a</varname> by one.</entry>
</row>
<row>
<entry>--$a</entry>
<entry>Pre-decrement</entry>
<entry>Decrements <varname>$a</varname> by one, then returns <varname>$a</varname>.</entry>
</row>
<row>
<entry>$a--</entry>
<entry>Post-decrement</entry>
<entry>Returns <varname>$a</varname>, then decrements <varname>$a</varname> by one.</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
Here's a simple example script:
<informalexample>
<programlisting role="php">
<![CDATA[
<?php
echo 'Post-increment:', PHP_EOL;
$a = 5;
var_dump($a++);
var_dump($a);
echo 'Pre-increment:', PHP_EOL;
$a = 5;
var_dump(++$a);
var_dump($a);
echo 'Post-decrement:', PHP_EOL;
$a = 5;
var_dump($a--);
var_dump($a);
echo 'Pre-decrement:', PHP_EOL;
$a = 5;
var_dump(--$a);
var_dump($a);
?>
]]>
</programlisting>
&example.outputs;
<screen>
<![CDATA[
Post-increment:
int(5)
int(6)
Pre-increment:
int(6)
int(6)
Post-decrement:
int(5)
int(4)
Pre-decrement:
int(4)
int(4)
]]>
</screen>
</informalexample>
<warning>
<para>
The increment and decrement operators have no effect on values
of type <type>bool</type>.
A <constant>E_WARNING</constant> is emitted as of PHP 8.3.0,
because this will implicitly cast the value to <type>int</type> in the future.
</para>
<para>
The decrement operator has no effect on values
of type <type>null</type>.
A <constant>E_WARNING</constant> is emitted as of PHP 8.3.0,
because this will implicitly cast the value to <type>int</type> in the future.
</para>
<para>
The decrement operator has no effect on non-
<link linkend="language.types.numeric-strings">numeric string</link>.
A <constant>E_WARNING</constant> is emitted as of PHP 8.3.0,
because a <classname>TypeError</classname> will be thrown in the future.
</para>
</warning>
<note>
<para>
Internal objects that support overloading addition and/or subtraction
can also be incremented and/or decremented.
One such internal object is <classname>GMP</classname>.
</para>
</note>
</para>
<sect2 xml:id="language.operators.increment.string">
<title>PERL string increment feature</title>
<warning>
<simpara>
This feature is soft-deprecated as of PHP 8.3.0.
The <function>str_increment</function> function should be used instead.
</simpara>
</warning>
<para>
It is possible to increment a non-
<link linkend="language.types.numeric-strings">numeric string</link>
in PHP. The string must be an alphanumerical ASCII string.
Which increments letters up to the next letter, when reaching the letter
<literal>Z</literal> the increment is carried to the value on the left.
For example, <code>$a = 'Z'; $a++;</code> turns <varname>$a</varname>
into <literal>'AA'</literal>.
</para>
<example>
<title>PERL string increment example</title>
<programlisting role="php">
<![CDATA[
<?php
echo '== Alphabetic strings ==' . PHP_EOL;
$s = 'W';
for ($n=0; $n<6; $n++) {
echo ++$s . PHP_EOL;
}
// Alphanumeric strings behave differently
echo '== Alphanumeric strings ==' . PHP_EOL;
$d = 'A8';
for ($n=0; $n<6; $n++) {
echo ++$d . PHP_EOL;
}
$d = 'A08';
for ($n=0; $n<6; $n++) {
echo ++$d . PHP_EOL;
}
?>
]]>
</programlisting>
&example.outputs;
<screen>
<![CDATA[
== Alphabetic strings ==
X
Y
Z
AA
AB
AC
== Alphanumeric strings ==
A9
B0
B1
B2
B3
B4
A09
A10
A11
A12
A13
A14
]]>
</screen>
</example>
<warning>
<para>
If the alphanumerical string can be interpreted as a
<link linkend="language.types.numeric-strings">numeric string</link>
it will be cast to an <type>int</type> or <type>float</type>.
This is particularly an issue with strings that look like a floating point
numbers written in exponential notation.
The <function>str_increment</function> function does not suffer from
these implicit type cast.
</para>
<example>
<title>Alphanumerical string converted to float</title>
<programlisting role="php">
<![CDATA[
<?php
$s = "5d9";
var_dump(++$s);
var_dump(++$s);
?>
]]>
</programlisting>
&example.outputs;
<screen>
<![CDATA[
string(3) "5e0"
float(6)
]]>
</screen>
<para>
This is because the value <literal>"5e0"</literal> is interpreted
as a <type>float</type> and cast to the value <literal>5.0</literal>
before being incremented.
</para>
</example>
</warning>
</sect2>
</sect1>
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