If you're not looking to duplicate the rest of the string, but instead just want the offset, in the spirit of the str*pos() functions, use strcspn()(PHP 5, PHP 7, PHP 8)
strpbrk — Search a string for any of a set of characters
   strpbrk() searches the string
   string for a characters.
  
string
       The string where characters is looked for.
      
charactersThis parameter is case sensitive.
   Returns a string starting from the character found, or false if it is
   not found.
  
Приклад #1 strpbrk() example
<?php
$text = 'This is a Simple text.';
// this echoes "is is a Simple text." because 'i' is matched first
echo strpbrk($text, 'mi'), PHP_EOL;
// this echoes "Simple text." because chars are case sensitive
echo strpbrk($text, 'S'), PHP_EOL;
?>
If you're not looking to duplicate the rest of the string, but instead just want the offset, in the spirit of the str*pos() functions, use strcspn()A little modification to Evan's code to use an array for the second parameter :
<?php
function strpbrkpos($s, $accept) {
  $r = FALSE;
  $t = 0;
  $i = 0;
  $accept_l = count($accept);
  for ( ; $i < $accept_l ; $i++ )
    if ( ($t = strpos($s, $accept[$i])) !== FALSE )
      if ( ($r === FALSE) || ($t < $r) )
        $r = $t;
    return $r;
}
?>