PHP按符號截取字符串的指定部分

function cut_str($str,$sign,$number){
   $array=explode($sign, $str);
   $length=count($array);
   if($number<0){
      $new_array=array_reverse($array);
      $abs_number=abs($number);
      if($abs_number>$length){
         return 'error';
      }else{
         return $new_array[$abs_number-1];
      }
   }else{
      if($number>=$length){
         return 'error';
      }else{
         return $array[$number];
      }
   }
}

$str='123/456/789/abc';
示例：
echo cut_str($str,'/',0); //輸出 123
echo cut_str($str,'/',2); //輸出 789
echo cut_str($str,'/',-1);//輸出 abc
echo cut_str($str,'/',-3);//輸出 456