On the scope of PHP variable and the problem of address reference _php tutorial

Concept of scope:

Variables can be declared anywhere in the PHP script, but declaring the location of a variable greatly affects the scope of the access variable. This accessible scope is called scope.

The main commonly used include: local variables, global variables, static variables.

1, local variables: is the variable declared within the function, he is stored in the memory stack, so the access speed quickly. Valid only within a function.

2. Global variables: In contrast to local variables, global variables can be accessed from anywhere in the program. You can recognize a global variable by adding the keyword global before the variable. Valid within the entire PHP file.

3, static variable: With static decoration only exists in the function scope of the variable, after the function execution ends its value does not disappear. Note: You cannot initialize after initialization, and you cannot use an expression to assign a value.

Copy the Code code as follows:
function test ()
{
Static $b =0;//declaration of a statically variable, which is not used inside a function if it is declared outside the function.

$b = $b +1;

Echo $b;
}
Test ();//This statement outputs a value of 1 $b
Test ();//This statement outputs a value of 2 $b

Note: Static $b =0 This assignment will only be performed when the variable is initialized for the first time.

Attached a: static members and static methods in the class, almost just call the time uniformly use the class name or self or parent plus: XXX, their scope and this same, but his statement is outside the method

The scope of the attached B:JS makes: aa= ' xxx ' with var; the global variable is declared outside the function (with or without the modifier var). Using var inside a function declares a local variable, and a global variable is not decorated with var.

Attached C: About references

PHP References: A reference to a variable, function, or object before adding &.php to the content of the same variable with a different name.

1. References to variables:

Copy the Code code as follows:
$a = "ABC";

$b =& $a;

echo $a;//output here: ABC

echo $b;//output here: ABC

$b = "EFG";

echo $a;//The value of $ A here becomes EFG so the output EFG

echo $b;//Output EFG here

2, the function of the address call

Copy the Code code as follows:
Function test (& $a)

{

$a = $a +100;

}

$b = 1;

echo $b;//Output 1

Test ($b); Here $b passed to the function is actually the memory address of the variable content of $b, by changing the value of $ A in the function can change the value of $b

echo "
";

echo $b;//Output 101

3. A reference to the function returns

Copy the Code code as follows:
function &test ()
{

Static $b =0;//declaration of a statically variable

$b = $b +1;

Echo $b;

return $b;

}

$a =test ();//This statement outputs a value of $b of 1

$a = 5;

$a =test ();//This statement outputs a value of $b of 2

$a =&test ();//This statement outputs a value of $b of 3

$a = 5;

$a =test ();//This statement outputs a value of $b of 6

Parsing: The use of $a=test () is actually not a function of the reference returned. Simply copy the return value of the function to $ A, without affecting the $b. There is no difference between this call and the normal call.

PHP Rules: $a =&test () is the way to get a reference to the function is returned. He points the memory address of the $b variable to the same place as the memory address of the $ A variable. That is equivalent to $a=& $b;

4. dereference

Copy the Code code as follows:
$a = 1;

$b =& $a;

unset ($a);

Echo $b;

Parse: unset a reference, just cancels the binding between the variable name and the contents of the variable, does not mean that the content is destroyed, its value is still real.

5. Global reference: When declaring a variable using the global $var, it is actually establishing a reference to the global variable. Global $val <=> $var =& $GLOBALS [' var '];

6. Object reference: In the method of the object, the object that $this called is the reference that invokes it

Note: In PHP, the point of the address is not implemented by the user itself, but through the Zend core implementation, the PHP reference is the "write copy" principle, that is, unless a write operation, a pointer to the same address of the variable or object will not be copied.

Copy the Code code as follows:
$a = 1;

$b = $a;

Both the $a and $b point to the same memory address, not $ A and $b occupy different memory.

If you execute a $a= "DSD" Now: the memory data pointed to by the $a and $b needs to be re-written, and the Zend core is automatically judged. Automatically generates a $ A copy of the data for $b and re-applies a piece of memory for storage.

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