Trick to rember the built in function.

Answer 1

no need to remember all. most of IDE or editor has auto complete / function suggestion feature.

Answer 2

trick...
the more you coffee.... the more you remember..

<<<focus on logic and algo for better career... no big company is not going to ask these shit functions>>>

as php functions are same as English language with snake case..

array_merge()
array_push()
array_explode()

try to connect functions with real life all is fine..