php字符串如何转为16进制字符串

/**
**字符串转16进制
**/
 public function String2Hex($string){
 $hex='';
 for ($i=0; $i < strlen($string); $i++){
 $hex .= dechex(ord($string[$i]));
 }
 return $hex;
 }
/**
**16进制转字符串
**/
 public function Hex2String($hex){
 $string='';
 for ($i=0; $i < strlen($hex)-1; $i+=2){
 $string .= chr(hexdec($hex[$i].$hex[$i+1]));
 }
 return $string;
 }
// example:
$hex = String2Hex("test sentence...");
// $hex contains 746573742073656e74656e63652e2e2e
echo Hex2String($hex);
// outputs: test sentenc

 public function String2Hex($string){
 $hex='';
// for ($i=0; $i < strlen($string); $i++){
// $hex .= dechex(ord($string[$i]));
// }
 $hex = bin2hex($string);
 return $hex;
 }