Is PHP gmp_random_range function from GMP extension cryptographically secure?

This is my first question, so excuse my clumsyness (and I am not a programmer - so even more excuses). It's not a problem but a question. I use PHP to write a software that generates random combinations of characters. For this I need arbitrary length integers random generation. So I use GMP extension and especially gmp_random_range(). But I need to prove with a link for example that it is cryptographically secure. Or at least it is enough random. GMP random functions use seed (gmp_random_seed ), and if I don't set a seed everything looks random enough. So, I suppose when I don't set the seed explicitly it takes it from some reliable random source. I could not find something clearly stating such thing. Any help is appreciated, thank you in advance.