不推荐: mysql_connect()

You can remove the warning by adding a '@' before the mysql_connect.

@mysql_connect('localhost','root','');

but as the warning is telling you, use mysqli or PDO since the mysql extension will be removed in the future.

That is because you are using PHP 5.5 or your webserver would have been upgraded to 5.5.0.

The mysql_* functions has been deprecated as of 5.5.0

Source

mysql_*, is officially deprecated as of PHP v5.5.0 and will be removed in the future.

Use mysqli_* function or pdo

Read Oracle Converting to MySQLi

Its just a warning that is telling you to start using newer methods of connecting to your db such as pdo objects

http://code.tutsplus.com/tutorials/php-database-access-are-you-doing-it-correctly--net-25338

The manual is here

http://www.php.net/manual/en/book.pdo.php

Warning "deprecated" in general means that you are trying to use function that is outdated. It doeasnt mean thaqt your code wont work, but you should consider refactoring.

In your case functons mysql_ are deprecated. If you want to know more about that here is good explanation already : Why shouldn't I use mysql_* functions in PHP?

Deprecated features in PHP 5.5.x

The original MySQL extension is now deprecated, and will generate E_DEPRECATED errors when connecting to a database. Instead, use the **MYSQLi or PDO_MySQL extensions.**

Syntax:

<?php
$connect = mysqli_connect('localhost', 'user', 'password', 'dbname');

Also, replace all ABC0 functions into mysqli_* functions

instead of

<?php
$connect = mysql_connect('localhost','root','');
mysql_select_db('dbname');
?>

There are a few solutions to your problem.

The way with MySQLi would be like this:

<?php
$connection = mysqli_connect('localhost', 'username', 'password', 'database');

To run database queries is also simple and nearly identical with the old way:

<?php
// Old way
mysql_query('CREATE TEMPORARY TABLE `table`', $connection);
// New way
mysqli_query($connection, 'CREATE TEMPORARY TABLE `table`');

Turn off all deprecated warnings including them from mysql_*:

<?php
error_reporting(E_ALL ^ E_DEPRECATED);

The Exact file and line location which needs to be replaced is "/System/Startup.php > line: 2 " error_reporting(E_All); replace with error_reporting(E_ALL ^ E_DEPRECATED);

<?php
$link = mysqli_connect('localhost','root','');
if (!$link) {
die('Could not connect to MySQL: ' . mysqli_error());
}
echo 'Connection OK'; mysqli_close($link);
?>

This will solve your problem.

This warning is displayed because a new extension has appeared. It suppouse that you still can use the old one but in some cases it´s impossible.

I show you how I do the connection with database. You need just change the values of the variables.

My connection file: connection.php

<?php
$host='IP or Server Name (usually "localhost") ';
$user='Database user';
$password='Database password';
$db='Database name';


//PHP 5.4 o earlier (DEPRECATED)
$con = mysql_connect($host,$user,$password) or exit("Connection Error");
$connection = mysql_select_db($db, $con);


//PHP 5.5 (New method)
$connection =  mysqli_connect($host,$user,$password,$db);
?>

The extension changes too when performing a query.

Query File: "example.php"

<?php
//First I call for the connection
require("connection.php");


// ... Here code if you need do something ...


$query = "Here the query you are going to perform";


//QUERY PHP 5.4 o earlier (DEPRECATED)
$result = mysql_query ($query) or exit("The query could not be performed");


//QUERY PHP 5.5 (NEW EXTENSION)
$result = mysqli_query ($query) or exit("The query could not be performed");
?>

This way is using MySQL Improved Extension, but you can use PDO (PHP Data Objects).

First method can be used only with MySQL databases, but PDO can manage different types of databases.

I'm going to put an example but it´s necessary to say that I only use the first one, so please correct me if there is any error.

My PDO connection file: "PDOconnection.php"

<?php
$hostDb='mysql:host= "Here IP or Server Name";dbname="Database name" ';
$user='Database user';
$password='Database password';


$connection = new PDO($hostDb, $user, $password);
?>

Query File (PDO): "example.php"

<?php
$query = "Here the query you are going to perform";
$result=$connection->$query;
?>

To finish just say that of course you can hide the warning but it´s not a good idea because can help you in future save time if an error happens (all of us knows the theory but if you work a lot of hours sometimes... brain is not there ^^ ).

Well, i just faced such message today when i moved to new hosting! anyway i have tried to change the "mySQL" to "mySQLi" but not working, so i have done this:

<?php
# FileName="Connection_php_mysql.htm"
# Type="MYSQL"
# HTTP="true"
# Turn off all error reporting
error_reporting(0);
$connect_myconn = "Database Connection";
$hostname_myconn = "localhost";
$database_myconn = "db name";
$username_myconn = "user name";
$password_myconn = "pass";
$myconn = mysql_connect($hostname_myconn, $username_myconn, $password_myconn) or die("<h1 style=margin:0;>A MySQL error has occurred.</h1><p><b>Your Query:</b> " . $connect_myconn . "<br /> <b>Error Number:</b> (" . mysql_errno() . ")</p>" . mysql_error());
mysql_select_db($database_myconn, $myconn);
?>

The trick is to set error reporting off :)

# Turn off all error reporting
error_reporting(0);

For PHP 7+ you can use this code instead:

ini_set('display_errors', 0);
ini_set('log_errors', 1);

Thanks

PDO class replaces these methods. Example for Mysql or MariaDB :

$BDD_SQL = new PDO('mysql:host='.BDD_SQL_SERVER.';dbname='.BDD_SQL_BASE.';charset=utf8',
BDD_SQL_LOGIN, BDD_SQL_PWD,
array(
PDO::ATTR_EMULATE_PREPARES => false,
PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION, //launch exception if error
PDO::ATTR_DEFAULT_FETCH_MODE=> PDO::FETCH_ASSOC
));

Source : PDO Class

If you have done your coding then

ini_set("error_reporting", E_ALL & ~E_DEPRECATED);

is good option but if you are in beginning then definitely you should use mysqli.