What is the difference between isset() and other is_*() functions (is_alpha(), is_number(), etc.)?
A.
isset() is a function call and is_*() are not function calls
B.
is_*() are language constructs and isset() is not a language construct
C.
c) isset() is a language construct and is_*() are not language constructs
D.
d) is_*() return a value whereas isset() does not