wpdb: use function_exists() instead of slower version_compare()

[DD32]

Wpdb's escaping code currently compares the PHP version to determine if a function will be available or not.

function_exists('mysql_real_escape_string');

is about twice as fast as the current code(on testing of 1,000 iterations):

version_compare( phpversion(), '4.3.0' );

[ryan]

That code isn't even be running since we return on the line before. But, yes, function_exists is better for such things.

[DD32]

Now why didnt i notice that? :)

[westi]

See [6366]

[DD32]

While this has been set to Invalid, Please keep it in mind in relation to #5455 which is suggesting to use real_escape again

[darkdragon]

I don't think this should be set to invalid.