php-doc-en/reference/array/functions/array-key-first.xml

<?xml version="1.0" encoding="utf-8"?>
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<refentry xml:id="function.array-key-first" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
 <refnamediv>
  <refname>array_key_first</refname>
  <refpurpose>Gets the first key of an array</refpurpose>
 </refnamediv>

 <refsect1 role="description">
  &reftitle.description;
  <methodsynopsis>
   <type>mixed</type><methodname>array_key_first</methodname>
   <methodparam><type>array</type><parameter>array</parameter></methodparam>
  </methodsynopsis>
  <para>
   Get the first key of the given <parameter>array</parameter> without affecting
   the internal array pointer.
  </para>
 </refsect1>

 <refsect1 role="parameters">
  &reftitle.parameters;
  <variablelist>
   <varlistentry>
    <term><parameter>array</parameter></term>
    <listitem>
     <para>
      An array.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

 <refsect1 role="returnvalues">
  &reftitle.returnvalues;
  <para>
   Returns the first key of <parameter>array</parameter> if the array is not empty;
   &null; otherwise.
  </para>
 </refsect1>

 <refsect1 role="examples">
  &reftitle.examples;
  <para>
   <example xml:id="array_key_first.example.basic">
    <title>Basic <function>array_key_first</function> Usage</title>
    <programlisting role="php">
<![CDATA[
<?php
$array = ['a' => 1, 'b' => 2, 'c' => 3];

$firstKey = array_key_first($array);

var_dump($firstKey);
?>
]]>
    </programlisting>
    &example.outputs;
    <screen>
<![CDATA[
string(1) "a"
]]>
    </screen>
   </example>
  </para>
 </refsect1>

 <refsect1 role="notes">
  &reftitle.notes;
  <tip>
    <simpara>
     There are several ways to provide this functionality for versions prior to PHP 7.3.0.
     It is possible to use <function>array_keys</function>, but that may be rather
     inefficient. It is also possible to use <function>reset</function> and <function>key</function>,
     but that may change the internal array pointer. An efficient solution, which does
     not change the internal array pointer, written as polyfill:
    </simpara>
   <informalexample>
    <programlisting role="php">
<![CDATA[
<?php
if (!function_exists('array_key_first')) {
    function array_key_first(array $arr) {
        foreach($arr as $key => $unused) {
            return $key;
        }
        return NULL;
    }
}
?>
]]>
    </programlisting>
   </informalexample>
  </tip>
 </refsect1>

 <refsect1 role="seealso">
  &reftitle.seealso;
  <simplelist>
   <member><function>array_key_last</function></member>
   <member><function>reset</function></member>
  </simplelist>
 </refsect1>

</refentry>

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