include(), require() will always read in the target file, even if the line it's on never executes.
If you want to conditionally include a file, use include().
The conditional statement won't affect the require().
However, if the line on which the require() occurs is not executed, neither will any of the code in the target file be executed.
If the file is not found by require(), it will cause a fatal error and halt the execution of the script.
If the file is not found by include(), a warning will be issued, but execution will continue.