Doubt regarding calling mb_send_mail()

[retrymp3]

Since the preloadme(void) constructor function will execute the nested functions when the library is loaded, why is there the need to call mb_send_mail()?
Is it to seem more authentic or is there some thing I missed.
If it does execute the functions when it is loaded, what is the relevance of the mail function in PHP and sendmail binary?

Also is it possible to override the mb_send_mail() in the hook. So that when LD_PRELOAD is set, we can call it and get the arbitrary execution.