How to avoid shared sessions when accessing one domain - php

Question

I am having problems regarding the shared sessions once the users logged in simultaneously to the website. When the last user logs in, every users views the account of the last user, which is not appropriate (security issue I guess). All users must login to the same website and must show an output for a specific user only.
I am using PHP code. Does anyone also experienced these scenario? here's the code
function validate_login(){
$userName= trim((isset($_POST['username']) === TRUE ? $_POST['username'] : ''));
$userPass= trim((isset($_POST['userpass']) === TRUE ? $_POST['userpass'] : ''));
$userNameArray = explode("#", $userName);
$userName = $userNameArray[0];
$compLogin = ((isset($userNameArray[1]) === TRUE ? $userNameArray[1]: ''));
$KeepAlive = new KeepAlive();
if(isset($userNameArray[1])){
if(!$this->Login->checkattempt('admin')){
$loginArray = array(
'login' => $userName,
'comp_login_string' => $compLogin,
'pass' => $cryptography->encryptPassword($userPass)
);
$userCredentials = $this->Login->getLogin($loginArray);
if(!empty($userCredentials)){
$_SESSION['userCredentials'] = $userCredentials[0];
header('Content-Type: application/json');
$_SESSION['employee_user'] = '0';
$_SESSION['emp_id'] = '0';
$_SESSION['page']['client_ip_address'] = $KeepAlive->get_client_ip();
$_SESSION['page']['last_trans_time'] = time();
$this->Login->clear_attempt_log($_POST, 'admin');
echo json_encode(array('id' => $userCredentials[0]['id']));
}
} else {
array_shift($loginArray);
$this->Login->attempt_log($_POST, 'admin');
header('Content-Type: application/json');
echo json_encode(array('id' => '-1'));
}
} else {
echo json_encode(array('id' => '-2'));
}
}