Type constraints like PHP

In a strongly typed language, a type constraint is a syntactic requirement, that is, when a variable is defined, the type must be specified and the type data can be stored later;

PHP is a weak type, which is characterized by the need not to specify a type for the variable, but also to store any type thereafter, but in PHP's new syntax, in some specific cases, for certain types, syntax constraints are also available.

• Specific occasions: form parametric of functions (methods)
• Specific types: Object types (class names), interface types (interface names), array types (arrays), function types (callable)

That is, you can set the type that must be used for the parameters of the function (method). Only objects, interfaces, arrays, and functions can be constrained.

functionf(类名 $p){}  // 要求参数只能使用该类的对象functionf(接口名 $p){// 要求该参数只能使用实现该接口的对象functionf(arrary $p){}  // 要求该参数只能使用数组functionf(callable $p)// 要求该参数只能是一个函数（方法），此时称之为回调函数（方法）

    
     classA{}functionf(A $p){}$obj = new A();f($obj);

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The above introduces PHP and other types of constraints, including aspects of the content, I hope that the PHP tutorial interested in a friend to help.

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