Without using regex, something like this should work for returning the string positions:
$html = "dddasdfdddasdffff";
$needle = "asdf";
$lastPos = 0;
$positions = array();
while (($lastPos = strpos($html, $needle, $lastPos))!== false) {
$positions[] = $lastPos;
$lastPos = $lastPos + strlen($needle);
}
// Displays 3 and 10
foreach ($positions as $value) {
echo $value ."<br />";
}