private static function highlight_yt($var)
    {
if (!function_exists('process_yt')) { 
function process_yt($a) { 
$arr = explode("=", $a);
$a = $arr[1];
return $a;
}
}
return preg_replace(array('#\[v\](.+?)\[/v\]#is'), array('<iframe width="340" height="176" src="http://www.youtube.com/embed/$1?rel=0" frameborder="0" allowfullscreen="true"></iframe>'), $var);
//return preg_replace(array('#\[v\](.+?)\[/v\]#is'), array('$1'), $var);
}

<?php
 
function youtube($string) {
return preg_replace(
'~\[v\](\d+)\[\/v\]~ius', 
'<iframe width="340" height="176" src="http://www.youtube.com/embed/$1?rel=0" frameborder="0" allowfullscreen="true"></iframe>', 
$string
);
}
 
print youtube('test[v]66666[/v]test[v]non_processed[/v][v]123456[/v]');