There are many ways to check PHP version running on your Linux VPS server.
1. From SSH:You can run following command via SSH
# php -v
2. phpinfo() function.You can simply create one phpinfo.php file and place below code:
<?
phpinfo();
?>
By browsing
http://www.yourdomain.com/phpinfo.php will give you version of PHP and also detailed list of PHP modules installed on your server.
Thanks!