session_set_save_handler is not working

I’ve been trying to set up saving sessions with session_set_save_handler() and i dont know what’s the problem, i have all my callable functions but sth is going wrong, here i post my code. btw i’m working in WP.

    <?php 
$root = dirname(dirname(dirname(dirname(__FILE__))));
if (file_exists($root.'/wp-load.php')) {
    require_once($root.'/wp-load.php');
}
if (file_exists($root.'/wp-config.php')) {
    require_once($root.'/wp-config.php');
}

global $wpdb;

//function called when session_start();
function _open() {
    print "Session opened.n";
    print "</br>";
    return true;
}

//called at the end of the page
function _close() {
    print "</br>";
    print "Session closed.n";
    print "</br>";
    return true;
}

//function called after session_start();
function _read($session_id) {
    global $wpdb;

    $session_id = mysql_real_escape_string($session_id);
    $record = $wpdb->get_var( "SELECT data FROM sessions_gm WHERE id='$session_id'");
    print "SELECT data FROM sessions_gm WHERE id='$session_id'";

    print "Session READ.n";
    print "</br>";
    print "El record:".$record;

     if($record)
    return $record;

    return '';
}

function _write($session_id, $data) {
    global $wpdb;
    $access = time();
    $session_id = mysql_real_escape_string($session_id);
    $access = mysql_real_escape_string($access);
    $data = mysql_real_escape_string($data);

    $session = $wpdb->get_row("SELECT * FROM sessions_gm WHERE id = '$session_id'");

    print "SELECT * FROM sessions_gm WHERE id = '$session_id'";
    print "La session:";
    print_r($session);

    print "</br>";
    print "Session value written.n";
    print "</br>";
    print "Sess_ID: $session_idn";
    print "</br>";
    print "Data: $datann";
    print "</br>";

    if($session == null)
    {
        $insert = $wpdb->insert( 
        'sessions_gm', 
        array( 
        'id' => $session_id, 
        'access' => $access,
        'data' => $data
        ), 
        array( 
        '%s', 
        '%d', 
        '%s'
        ) 
        );

        if($insert!=false)
        {
            print "Se inserto.";
            return true;
        }
    }
    else
    {
        $update = $wpdb->update( 
            'sessions_gm', 
            array( 
            'access' => $access,    // string
            'data' => $data // integer (number) 
            ), 
            array( 'id' => $session_id ), 
            array( 
            '%d',   // value1
            '%s'    // value2
            ), 
            array( '%s' ) 
        );

        if($update!=false)
        {
            print "Se updateo.";
            return true;
        }
    }

    return false;
}

// called when session_destroy();
function _destroy($session_id) {
    global $wpdb;
    $session_id = mysql_real_escape_string($session_id);
    print "</br>";
    print "Session destroy called.n";
    print "</br>";
    $delete=$wpdb->query( "DELETE FROM sessions_gm WHERE id = '$session_id'");
    if($delete!=false)
    {
        print "Se elimino";
        return true;
    }
    return false;
}

function _gc($maxlifetime) {
    print "</br>";
    print "Session garbage collection called.n";
    print "</br>";
    print "Sess_maxlifetime: $maxlifetimen";
    print "</br>";
   $old = time() - $max;
   $old = mysql_real_escape_string($old);
   $delete=$wpdb->query( "DELETE FROM sessions_gm WHERE access < '$old'");

     if($delete!=false)
    {
        print "Se elimino las pasadas.";
        return true;
    }
    return false;
}

var_dump(is_callable("_open"));
var_dump(is_callable("_close"));
var_dump(is_callable("_read"));
var_dump(is_callable("_write"));
var_dump(is_callable("_destroy"));
var_dump(is_callable("_gc"));


if (session_set_save_handler("_open", "_close", "_read", "_write", "_destroy", "_gc")) {
    die('Works fine');
}
else {
    die('Couldn't set session handler');
} 

session_set_save_handler("_open", "_close", "_read", "_write", "_destroy", "_gc");
session_start();

$_SESSION['Username'] = 'clau';  
print_r($_SESSION);?>

So for some reason in my if:

        if (session_set_save_handler("_open", "_close", "_read", "_write", "_destroy", "_gc")) {
    die('Works fine');
}
else {
    die('Couldn't set session handler');}

it’s returning Couldnt set session handler.
Thanks for the help! 😀