Essential Books

Nullable type hints

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Parameters

Nullable type hint was added in PHP 7.1 using the ? operator before the type hint. 

function f(?string $a) {}
function g(string $a) {}

f(null); // valid
g(null); // TypeError: Argument 1 passed to g() must be of the type string, null given

Before PHP 7.1, if a parameter has a type hint, it must declare a default value null to accept null values. 

function f(string $a = null) {}
function g(string $a) {}

f(null); // valid
g(null); // TypeError: Argument 1 passed to g() must be of the type string, null given

Return values

In PHP 7.0, functions with a return type must not return null.

In PHP 7.1, functions can declare a nullable return type hint. However, the function must still return null, not void (no/empty return statements). 

function f() : ?string {
    return null;
}

function g() : ?string {}
function h() : ?string {}

f(); // OK
g(); // TypeError: Return value of g() must be of the type string or null, none returned
h(); // TypeError: Return value of h() must be of the type string or null, none returned
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Type hinting:
* Type hinting
* Type hinting classes and interfaces
* Type hinting scalar types arrays and callables
* Nullable type hints
* Type hinting generic objects
* Type Hinting No ReturnVoid
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0 Getting started
1 Variables
2 Arrays
3 Functional programming
4 Types
5 Autoloading primer
6 Exception handling and error reporting
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37 Object serialization
38 PHPDoc
39 Contributing to PHP Manual
40 String parsing
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46 Type juggling and non-strict comparison issues
47 Security
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57 Installing PHP on Windows
58 DateTime class
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62 Installing on Linux / Unix
63 Contributing to PHP Core
64 Coding conventions
65 Using MongoDB
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68 Unicode Support
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84 Outputting the value of a variable
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92 Comments
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100 Docker deployment
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104 Secure remember me
105 PHP mysqli
106 PHP built-in server
107 How to break down an URL
108 PSR
109 Contributors