<?php

$mysql_host='localhost';
$mysql_user='root';
$mysql_password='';
$mysql_db='als';
$con = @mysql_connect($mysql_host,$mysql_user,$mysql_password) or             die(mysql_error());
@mysql_select_db($mysql_db) or die(mysql_error());

?>

$mysql_host='localhost';
$mysql_user='root';
$mysql_password='';
$mysql_db='als';
$con= mysqli_connect($mysql_host,$mysql_user,$mysql_password, $mysql_db) or die("Error " . mysqli_error($con));

<?php
$mysqli = new mysqli('localhost', 'root', '');
if ($mysqli->connect_errno) {
    trigger_error('query failed: '.$mysqli->connect_error, E_USER_ERROR);
}

$result = $mysqli->query('SHOW databases')
    or trigger_error('connect failed: '.join(',', $mysqli->error_list), E_USER_ERROR);

foreach( $result as $row ) {
    echo join(', ', $row), "<br />rn";
}

<?php
$pdo = new PDO('mysql:host=localhost;charset=utf8', 'root', '', array(
    PDO::MYSQL_ATTR_DIRECT_QUERY => false,
    PDO::ATTR_ERRMODE=>PDO::ERRMODE_EXCEPTION
));

foreach( $pdo->query('SHOW DATABASES', PDO::FETCH_NUM) as $row ) {
    echo $row[0], "<br />rn";
}