About PHP types 
In PHP coding, the following types considered as primitives are supported:
SCALAR TYPES
Four SCALAR TYPES:
a) boolean, (bool),
b) integer, (int),
c) float aka double and
d) string, (str).
COMPOUND TYPES
Four COMPOUND TYPES:
e) array, (arr),
f) object, (obj),
g) callable, (call) and
h) iterable, (iter).
SPECIAL TYPES
Two SPECIAL TYPES:
i) resource, (res) and
j) NULL or null.
PSEUDO TYPES
Five PSEUDO TYPES:
a) mixed, (mix),
b) number, (nbr),
c) callback aka callable, (call),
d) array|object, (arrobj) and
e) void, (void).
The abbreviated names in parentheses are not official, rather, they are only used to simplify parts of this tutorial.
We will study each of these types in due course.