Fix unnecessary PHP warnings

[WM-Programator-3]

Hello,

Not checking if file exists in class lib/OpenPayU/Oauth/Cache/OauthCacheFile.php causes PHP to throw errors.

Using @ doesn't suppress the errors if custom error handler is used, and since PHP 8, it is impossible to detect if @ was used in custom error handlers because of a "fix" of a bug that was widely accepted as a proper behaviour by the PHP community.

There is a simple fix to this problem - use PHP function file_exists() in get(), set() and invalidate() methods, which is the proper way of doing things in modern PHP.

The function is available since PHP 4 while openpayu_php requires PHP >= 5.3, which means there is no issue with compatibility.

Otherwise, users themselves would need to write some code specifically to suppress the warnings in you class in their error handlers.

Thanks.

[WM-Programator-3]

Note:

Example of fixed methods:

    public function get($key)
    {
        return !file_exists($this->directory . md5($key)) ? null : unserialize(file_get_contents($this->directory . md5($key)));
    }

    public function invalidate($key)
    {
        return !file_exists($this->directory . md5($key)) || unlink($this->directory . md5($key));
    }