Fix errors found by static code analysis to remove error flags raised by the IDE

[andergmartins]

Expected Behavior

When opening the plugin's code in my IDE (Cursor), I expect not to see error flags about classes that are not found.

Current Behavior

The class MA_Wpengine_Integration is flagged with an error in the method flushWPECache because the class \WpeCommon does not exist on the project. The flag is correct since that class is only defined when that 3rd party plugin is installed, but we have options to keep its functionality and still avoid those flags.

Possible Solution

Refactor the method flushWPECache using method_exists instead of function_exists and using call_user_func instead of mentioning the class name directly. Something like the following snippet

if (method_exists('WpeCommon', 'purge_memcached')) {
    call_user_func(['WpeCommon', 'purge_memcached']);
}

Using that snippet, we mention the class name as a string, not triggering the errors, considering it will only be called if the class is also defined.

[andergmartins]

I've found a similar issue on src/core/Classes/Utils.php:

Undefined function et_get_theme_version

Use call_user_func on that function.

Undefined constant ELEMENTOR_PRO_VERSION

Use constant('ELEMENTOR_PRO_VERSION') instead of the constant directly.

Undefined constant WPSEO_VERSION

Also use constant(.....