<?php
$dirs[] = array('file'=>'dirname1', 'is_dir'=> 1 , 'size'=> 0 , 'mtime'=> 1117087179 , 'mode'=>'');
$dirs[] = array('file'=>'arrow1.gif', 'is_dir'=> 0 , 'size'=> 84861 , 'mtime'=> 1117101118 , 'mode'=> 33206 );
$dirs[] = array('file'=>'x.jpg', 'is_dir'=> 0 , 'size'=> 12861 , 'mtime'=> 1117101119 , 'mode'=> 33206 );
$dirs[] = array('file'=>'arrow2.jpg', 'is_dir'=> 0 , 'size'=> 112861 , 'mtime'=> 1117101119 , 'mode'=> 33206 );
$dirs[] = array('file'=>'dirname2', 'is_dir'=> 1 , 'size'=> 0 , 'mtime'=> 1117087149 , 'mode'=>'');
$dirs[] = array('file'=>'z1.jpg', 'is_dir'=> 0 , 'size'=> 12861 , 'mtime'=> 1117101119 , 'mode'=> 33206 );

$content = print_r($dirs, true);
echo 'Before:<br>';
echo '<pre>';
echo $content;
echo '</pre>';
echo '<hr>';

$sorted = udfSort($dirs, 'file');

$content = print_r($dirs, true);
echo 'After sort by <b>\'file\'</b>:<br>';
echo '<pre>';
echo $content;
echo '</pre>';

// ------------------------------------------------------------------------
// ------------------------------------------------------------------------

// user defined function (sort ASC)
function udfSort(&$array, $index)
{
    $compare_func = create_function('$a, $b', 'return ($a[\''.$index.'\']==$b[\''.$index.'\'])? 0: (($a[\''.$index.'\']>$b[\''.$index.'\'])? 1: -1);');
    
    if (!function_exists($compare_func))
    {
        return false;
    }
    
    return usort($array, $compare_func);
}
?>

Before:

dirname1
arrow1.gif
x.jpg
arrow2.jpg
dirname2
z1.jpg

After sort by 'file':

arrow1.gif
arrow2.jpg
dirname1
dirname2
x.jpg
z1.jpg