Create a file named config.php where you will connect to the database. Put the following code in it:

$mysqli = new mysqli("localhost", "username", "password", "db");
if ($mysqli->connect_errno) {
    echo "Failed to connect to MySQL: (" . $mysqli->connect_errno . ") " . $mysqli->connect_error;
}
$bd = mysql_connect("localhost", "username", "password") or die(mysql_error());
mysql_select_db("db",$bd) or die(mysql_error());
?>

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Facebook have it’s own like system which pretty good. This post will tell you how to create a "Like system" like that of Facebook. All you have to need is jQuery, MySQL Database and PDO PHP extension. This technic is partially used in my social network, Open. See the demo there in the social network. But you have to signup.

<div>
  <span style="color: red; font-family: inherit;">The table should be like this:</span>
</div>

$user=$_GET[‘user’];
$sql=mysql_query("SELECT * FROM users WHERE user='".$user."‘");

http://example.com/user.php?user=subin

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SELECT * FROM `fdposts` WHERE user IN (SELECT friend FROM `fdfriends` WHERE user=’subins2000′)

The above code will select rows in table fdposts where user is the value of friend row in the table fdfriends.
As you can see it’s very simple. Here’s another example :

SELECT * FROM `posts` WHERE user IN (SELECT name FROM `members` WHERE user=’subins2000′)

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If you want to run code in PHP file do as below.
Open your PHP file.
Add these lines in the file.

mysql_query("sql code here");

NOTE – Replace the red line with the SQL code.