convert hex to string and vice versa

function str_to_hex($string){
$hex='';
for ($i=0; $i < strlen($string); $i++){
$hex .= dechex(ord($string[$i]));
}
return $hex;
}

/* Convert hex to string */

function hex_to_str($hex){
$string='';
for ($i=0; $i < strlen($hex)-1; $i+=2){
$string .= chr(hexdec($hex[$i].$hex[$i+1]));
}
return $string;
}

// example :

$hex = str_to_hex("test sentence...");
// $hex contains 746573742073656e74656e63652e2e2e

print hex_to_str($hex);
// outputs: test sentence...