PHP variable scope, PHP global variables and static variables

$a = 1;
include 'b.inc';
?>

Here variable $a will take effect in the include file b.inc. In user-defined functions, a local function scope will be introduced. Any variables used inside a function will be limited to the scope of the local function by default and are local variables at this time. ​ PHP global variables must be declared global when used in functions. Variables declared using global in a function are global variables and can be used outside the function.

Note: When declaring a variable globally, you cannot directly assign a value to the variable. You need to declare it first and then assign it.

In the global scope, global variables can also be accessed through $GLOBALS. There is no need to use the global keyword to access global variables within a function. $GLOBALS is an associative array, each variable is an element, the key name corresponds to the variable name, and the value corresponds to the variable's content. $GLOBALS exists in the global scope because $GLOBALS is a superglobal variable.

Constants can be defined and accessed anywhere regardless of the scope of the variable;

Another important feature of variable scope is static variables. PHP static variables only exist in the local function scope, but their values ​​are not lost when the program execution leaves this scope. Static variables are only initialized on the first call. They can be assigned values ​​when declared, but cannot be expression values. Simple example of php static variables Assigning it with the result of an expression in a declaration will result in a parsing error.

When a reference (a variable or object with &) is assigned to a static variable, the reference is not stored statically, and the value of the static variable is not remembered the second time the function is called. Similarly, when a reference (variable or object with &) is assigned to a global variable, the change of this variable has no effect outside the function, and the scope is only within the function.