PHP variable / function / class names using special characters

Question

I understand that the underscore _ is an acceptable character for naming variables / functions / classes etc. However I was wondering if there are any other special

Answer 1

If you want to structure names by combining elements (for example functions, and database column or tables names), you could use a double underscore as a separator:

social_security_number__check(),
musical_instrument__tune(),
vehicle__insert()

or

check__social_security_number(),
tune__musical_instrument(),
insert__vehicle()