php数据库连接功能

I am trying to connect to mySQL database using this code, but I always get an error saying

Fatal error: Uncaught Error: Call to undefined function mysql_connect()

here is my code;

<?php

        $servername = "";
        $user = "";
        $pwd = "";
        $dbname = "";


    function connexion()
        {
            global $servername, $user, $pwd, $dbname;
            $db=mysql_connect($servername,$user,$pwd) or die("Database connection failed: ".mysql_error());
            mysql_select_db($dbname,$db);
        }
?>

The point is to call for the function connecxion() after the user enter the right inputs. Can you tell me what I am doing wrong please?

Use PDO (PHP Data Object ) its secure

<?php
 try {
    $yourHandler = new PDO('mysql:dbhost=127.0.0.1;dbname=yourdbname;', 'user', 'pass'); //creates a new instance of the PDO database class and passes it to your handler and creates database connection 
    $yourHandler->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION); //sets the error mode so you can catch any errors that may occur
}  catch (PDOException $e) {
     echo $e->getMessage(); //echoes the errors
     die(); //kills the page 
}
?>