function extract_only_special_characters($s) {
$len = strlen($s);
$special_characters = "";
for ($i = 0; $i < $len; $i++) {
$ch = $s[$i];
if (!ctype_alnum($ch) && $ch != ' ') {
$special_characters = $special_characters . $ch;
}
}
return $special_characters;
}
$s = "c++14@vb.NET&%java*() php <>/python 3.7.3";
echo extract_only_special_characters($s);
/*
run:
++@.&%*()<>/..
*/