php-doc-en/reference/array/functions/usort.xml

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  <refentry id="function.usort">
   <refnamediv>
    <refname>usort</refname>
    <refpurpose>
     Sort an array by values using a user-defined comparison function
    </refpurpose>
   </refnamediv>
   <refsect1>
    <title>Description</title>
     <methodsynopsis>
      <type>void</type><methodname>usort</methodname>
      <methodparam><type>array</type><parameter>array</parameter></methodparam>
      <methodparam><type>callback</type><parameter>cmp_function</parameter></methodparam>
     </methodsynopsis>
    <para>
     This function will sort an array by its values using a
     user-supplied comparison function.  If the array you wish to sort
     needs to be sorted by some non-trivial criteria, you should use
     this function.
    </para>
    <para>
     The comparison function must return an integer less than, equal
     to, or greater than zero if the first argument is considered to
     be respectively less than, equal to, or greater than the
     second. 
     <note>
      <para>
       If two members compare as equal, their order in the sorted array is undefined.
       Up to PHP 4.0.6 the user defined functions would keep the original order for
       those elements, but with the new sort algorithm intruduced with 4.1.0 this
       is no longer the case as there is no solution to do so in an efficient way.
      </para>
     </note>
    </para>
    <para>
     <example>
      <title><function>usort</function> example</title>
      <programlisting role="php">
<![CDATA[
function cmp ($a, $b) {
    if ($a == $b) return 0;
    return ($a > $b) ? -1 : 1;
}

$a = array (3, 2, 5, 6, 1);

usort ($a, "cmp");

while (list ($key, $value) = each ($a)) {
    echo "$key: $value\n";
}
]]>
      </programlisting>
     </example>
    </para>
    <para>
     This example would display:
    </para>
    <para>
     <screen>
<![CDATA[
0: 6
1: 5
2: 3
3: 2
4: 1
]]>
     </screen>
    </para>
    <note>
     <para>
      Obviously in this trivial case the <function>rsort</function>
      function would be more appropriate.
     </para>
    </note>
    <para>
     <example>
      <title>
       <function>usort</function> example using multi-dimensional array
      </title>
      <programlisting role="php">
<![CDATA[
function cmp ($a, $b) {
    return strcmp($a["fruit"], $b["fruit"]);
}

$fruits[0]["fruit"] = "lemons";
$fruits[1]["fruit"] = "apples";
$fruits[2]["fruit"] = "grapes";

usort($fruits, "cmp");

while (list ($key, $value) = each ($fruits)) {
    echo "\$fruits[$key]: " . $value["fruit"] . "\n";
}
]]>
      </programlisting>
     </example>
    </para>
    <para>
     When sorting a multi-dimensional array, $a and $b contain
     references to the first index of the array.
    </para>
    <para>
     This example would display:
    </para>
    <para>
     <screen>
<![CDATA[
$fruits[0]: apples
$fruits[1]: grapes
$fruits[2]: lemons
]]>
     </screen>
    </para>

    <para>
     <example>
      <title>
       <function>usort</function> example using a member function of an object
      </title>
      <programlisting role="php">
<![CDATA[
class TestObj {
    var $name;

    function TestObj($name)
    {
        $this->name = $name;
    }

    /* This is the static comparing function: */
    function cmp_obj($a, $b)
    {
        $al = strtolower($a->name);
        $bl = strtolower($b->name);
        if ($al == $bl) return 0;
        return ($al > $bl) ? +1 : -1;
    }
}

$a[] = new TestObj("c");
$a[] = new TestObj("b");
$a[] = new TestObj("d");

uasort($a, array ("TestObj", "cmp_obj"));

foreach ($a as $item) {
    print $item->name."\n";
}
]]>
      </programlisting>
     </example>
    </para>
    <para>
     This example would display:
    </para>
    <para>
     <screen>
b
c
d
     </screen>
    </para>

    <para>
     See also <function>uasort</function>,
     <function>uksort</function>, <function>sort</function>,
     <function>asort</function>,
     <function>arsort</function>,<function>ksort</function>,
     <function>natsort</function>, and <function>rsort</function>.
    </para>
   </refsect1>
  </refentry>

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