To remove all characters from string $b that exist in string $a:
$a="ABC";
$b="teAsBtC";
echo str_replace(str_split($a),'',$b);
Output: test
To remove all characters from string $b that don't exist in string $a:
$a="ABC";
$b="teAsBtC";
echo str_replace(str_split(str_replace(str_split($a),'',$b)),'',$b);
Output: ABC