About PHP types 
In PHP coding, the following types considered as primitives are supported:
SCALAR TYPES
Four SCALAR TYPES:
a) bool,
b) int,
c) flat, (floating-point number aka double )
and
d) string
COMPOUND TYPES
Four COMPOUND TYPES:
e) array,
f) object,
g) callable
Callbacks can be denoted by the callable type declaration.
and
h) iterable
Iterable can be used as a parameter type to indicate that a function requires a set of values, but does not care about the form of the value set since it will be used with foreach.
SPECIAL TYPES
Two SPECIAL TYPES:
i) resource
and
j) NULL
PSEUDO TYPES
Five PSEUDO TYPES:
a) mixed,
b) number,
c) callback,
d) array|object
and
e) void
| Type |
Description |
Version |
| Class/interface name |
The value must be an instanceof the given class or interface
|
|
| self |
The value must be an instanceof the same class as the one
in which the type declaration is used.
Can only be used in classes.
|
|
| parent |
The value must be an instanceof the parent of the class
in which the type declaration is used.
Can only be used in classes.
|
|
| array |
The value must be an array.
|
|
| callable |
The value must be a valid callable.
Cannot be used as a class property type declaration.
|
|
| bool |
The value must be a boolean value.
|
|
| float |
The value must be a floating point number.
|
|
| int |
The value must be an integer.
|
|
| string |
The value must be a string.
|
|
| iterable |
The value must be either an array or an instanceof Traversable.
|
PHP 7.1.0 |
| object |
The value must be an object.
|
PHP 7.2.0 |
mixed
union |
The value can be any value.
equivalent to the union type:
array|bool|callable|int|float |object|resource|string|null
|
PHP 8.0.0 |
| php.net |
An union type declaration accepts values of multiple different types, rather than a single one.
union type is specified using the syntax T1|T2|....
union type is available as of PHP 8.0.0.
The null type is supported as part of unions, such that T1|T2|null can be used to create a nullable union.
The existing ?T notation is considered a shorthand for the common case of T|null.
null cannot be used as a standalone type.
The false literal type is supported as part of unions, and is included as for historical reasons many internal functions return false instead of null for failures.
false cannot be used as a standalone type (including nullable standalone type).
As such, all of false, false|null and ?false are not permitted.
The true literal type does not exist.
To catch simple bugs in union type declarations, redundant types that can be detected without performing class loading will result in a compile-time error.
This includes:
1. Each name-resolved type may only occur once.
Types such as int|string|INT result in an error.
2. If bool is used, false cannot be used additionally.
3. If object is used, class types cannot be used additionally.
4. If iterable is used, array and Traversable cannot be used additionally.
This does not guarantee that the type is “minimal”, because doing so would require loading all used class types.
For example, if A and B are class aliases, then A|B remains a legal union type, even though it could be reduced to either A or B.
Similarly, if class B extends A {}, then A|B is also a legal union type, even though it could be reduced to just A.
We will study each of these types in due course.