Codeigniter error: Call to undefined function mysql_pconnect()

Thanks to Anant

I come to a conclusion:

I completely changed my old database.php file in config folder with the new one:

From:

$db['default']['hostname'] = 'localhost';
$db['default']['username'] = '';
$db['default']['password'] = '';
$db['default']['database'] = '';
$db['default']['dbdriver'] = '';
$db['default']['dbprefix'] = '';
$db['default']['pconnect'] = TRUE;
$db['default']['db_debug'] = TRUE;
$db['default']['cache_on'] = FALSE;
$db['default']['cachedir'] = '';
$db['default']['char_set'] = 'utf8';
$db['default']['dbcollat'] = 'utf8_general_ci';
$db['default']['swap_pre'] = '';
$db['default']['autoinit'] = TRUE;
$db['default']['stricton'] = FALSE;

To:

$db['default'] = array(
    'dsn'   => '',
    'hostname' => '',
    'username' => '',
    'password' => '',
    'database' => '',
    'dbdriver' => 'mysqli',
    'dbprefix' => '',
    'pconnect' => FALSE,
    'db_debug' => (ENVIRONMENT !== 'production'),
    'cache_on' => FALSE,
    'cachedir' => '',
    'char_set' => 'utf8',
    'dbcollat' => 'utf8_general_ci',
    'swap_pre' => '',
    'encrypt' => FALSE,
    'compress' => FALSE,
    'stricton' => FALSE,
    'failover' => array(),
    'save_queries' => TRUE
);

And the error is gone!

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.

You're using the deprecated 'mysql' dbdriver. Locate the config/database.php file and change dbdriver to use 'mysqli' :

$db['default']['dbdriver'] = 'mysqli';