php隐藏手机号码中间几位数

function sub_right($string, $strlen = '50'){ 
    $tmpstr = ""; 
    $lengs =strlen($string)-3; 
    if($lengs<=10)$strlen=$lengs-4; 
    //$tmpstr .= substr($string, 0, $strlen); 
    //print_r($lengs); 
    for($i = 0; $i < strlen($string); $i++) { 
        if($i<$strlen){ 
            if(ord(substr($string, $i, 1)) > 0xa0) { 
                $tmpstr .= substr($string, $i, 2); 
                $i++; 
            } else 
                $tmpstr .= substr($string, $i, 1); 
        }else{ 
            if($i < $lengs){ 
                $tmpstr .="*"; 
            } 
        } 
    } 
    $tmpstr .= substr($string,-3); 
    return $tmpstr; 
 
} 
$str="13029942739"; 
echo sub_right($str,3);