<?php
$array = array(',', '.', '<');
$string = 'fdsg.gsdfh<dsf<g,gsd';

$search = implode($array); $last = 0; $out = ''; $pos = array();
foreach ($array as $char) $pos[$char] = array();

while (($len = strcspn($string, $search)) !== strlen($string)) {
    $last = ($pos[$string[$len]][] = $last + $len) + 1;
    $out .= substr($string, 0, $len);
    $string = substr($string, $len+1);
}
$out.=$string;

echo $out."\n".print_r($pos,true);

fdsggsdfhdsfggsd
Array
(
    [,] => Array
        (
            [0] => 16
        )

    [.] => Array
        (
            [0] => 4
        )

    [<] => Array
        (
            [0] => 10
            [1] => 14
        )

)