Variables are an essential thing in PHP programming. Variables in PHP are divided into global variables and private variables. Let me share some of my understanding and usage of PHP variables for your reference.
What aspects would you pay attention to when defining variables and constants? You may think:
•How to define variables, and how is it different from languages such as C#?
•Are variables case sensitive?
•Are there other important PHP variables?
•Are constants and variables defined the same?
Let’s tell them separately.
1. How to define variables, and how is it different from languages such as C#?
Variables in PHP are represented by a dollar sign followed by the variable name. Variable names are case-sensitive. For example:
Note, one thing that needs to be explained is that since PHP4, the concept of reference assignment has been introduced, which is actually similar to references in most languages, but I think the most similar one is C/C++ because it also uses the "&" symbol.
For example:
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1
2 $foo = 'Bob'; // Assign 'Bob' to foo<🎜>
3 $bar = &$foo; // Referenced through $bar. Note the & symbol <🎜>
4 $bar = "My name is $bar"; // Modify $bar<🎜>
5 echo $bar;<🎜>
6 echo $foo; // $foo has also been modified.<🎜>
7 ?>
Like other languages, only variables with variable names can be referenced
To put it bluntly, variable variables in PHP are to parse the value of a variable into a variable name and read the value of that variable name. Example:
$a = "China"; //Variable a
$b = "a"; //Variable b
$China = "I'm Chinese !"; //Variable China
$f = "b"; //Variable f
echo $a." "; //Output China
echo $$a." "; //Output I'm Chinese --If you want to parse it as a variable variable, you must add an extra $ symbol in front
$a = "f"; //Change the name pointed to by the variable (here is the application of variable variables)
echo $$a." "; //Output b after passing the variable f pointed to above
$a = "b"; //Same as above
echo $$a."
"; //Output a
echo $b." "; //output a
echo $$b." "; //output b
echo $$$b."
"; //Output a
echo $f." "; //output b
echo $$f." "; //output a
echo $$$f." "; //output b
echo $$$$f."
"; //Output a
$$a = "China"; //The last one has changed the variable to b. The so-called $$a=$b is the changed value of $b
echo $b." "; //Output China
echo $$b; //Output I'm Chinese
?>
Note: Mutable variables cannot be applied to $this and superglobal variables (the scope of PHP variables is different from other high-level programming languages. See the code)
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"; //Output abc
Echo $abc; //Output def
echo " ";
Function show()
{
global $name; //global here is not set as a global variable. Instead quote
echo $name." "; //Output man
}
Function showtwo()
{
//global $name;
//echo $name." ";
echo $GLOBALS['name']; //Super global variable array
}
show();
Showtwo();
?>
Variable function:
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代码如下
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function b()
{
echo "这是B";
}
function c($name = "China") //设默认值
{
echo "这是$name";
}
$a = 'b';
$a(); //找值所在的函数
$a = 'c';
$a(); ?>
function b()
{
echo "This is B";
}
function c($name = "China") //Set default value
{
echo "This is $name";
}
$a = 'b';
$a(); //Function to find the value
$a = 'c';
$a(); ?>
foreach($_POST as $key=>$value)
//print_r($_POST);
$$key = $value;
}
//extract($_POST); //Import variables from the array into the current symbol table --Find the PHP manual by yourself
echo $name." ";
echo $pwd." ";
echo $tag." ";
?>
Variable scope .
Variable scope
The scope of a variable is the context in which it is defined (Translator: To put it bluntly, its effective scope). Most PHP variables have only a single scope. This single scope span also includes files introduced by include and require. Example:
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代码如下
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$a = 1;
include "b.inc";
?>
$a = 1;
include "b.inc";
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$a = 1; /* global scope */
function Test()
{
echo $a; /* reference to local scope variable */
}
Test();
?>
?>
This variable $a will take effect in the include file b.inc. However, in user-defined functions, a local function scope will be introduced. Any variables used inside a function will be restricted to the local function scope by default. Example:
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$a = 1; /* global scope */
function Test()
{
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$a = 1;
$b = 2;
function Sum()
{
global $a, $b;
$b = $a + $b;
}
Sum();
echo $b;
?>
echo $a; /* reference to local scope variable */
}
Test();
?>
This script will produce no output because the echo statement refers to a local version of the variable $a, and it is not assigned a value within this scope. You may notice that PHP's global variables are a little different from C language. In C language, global variables automatically take effect in functions unless overridden by local variables. This can cause problems, as someone might carelessly change a global variable. Global variables in PHP must be declared global when used in functions.
The global keyword
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$a = 1;
$b = 2;
function Sum()
{
$GLOBALS["b"] = $GLOBALS["a"] + $GLOBALS["b"];
}
Sum();
echo $b;
?>
First, an example of using global:
Example 12-1. Using global
The output of the above script will be "3". Global variables $a and $b are declared in the function, and all reference variables of any variable will point to the global variables. PHP has no limit on the maximum number of global variables that a function can declare.
The second way to access variables in the global scope is to use a special PHP custom $GLOBALS array. The previous example can be written as:
Example 12-2. Using $GLOBALS instead of global
In the $GLOBALS array, each variable is an element, the key name corresponds to the variable name, and the value variable content. $GLOBALS exists in the global scope because $GLOBALS is a superglobal variable. The following example shows the use of superglobal variables:
Example 12-3. Example demonstrating superglobal variables and scope
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function test_global()
{
// 大多数的预定义变量并不 "super"，它们需要用 'global' 关键字来使它们在函数的本地区域中有效。
global $HTTP_POST_VARS;
function test_global()
{
// Most predefined variables are not "super", they require the 'global' keyword to make them available in the local scope of the function.
global $HTTP_POST_VARS;
print $HTTP_POST_VARS['name'];
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function Test ()
{
$a = 0;
echo $a;
$a++;
}
?>
// Superglobals are valid in any scope, they do not require a 'global' declaration. Superglobals were introduced in PHP 4.1.0.
Print $_POST['name'];
}
?>
Use static variables
Another important feature of variable scope is static variables. Static variables only exist in the local function scope, but their values are not lost when program execution leaves this scope. Take a look at the example below:
Example 12-4. Demonstrates the need for static variables
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function Test ()
{
$a = 0;
echo $a;
$a++;
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function Test()
{
static $count = 0;
$count++;
echo $count;
if ($count < 10) {
Test ();
}
$count--;
}
?>
}
?>
This function is not very useful because it sets the value of $a to 0 and prints "0" every time it is called. $a++, which increments a variable by one, has no effect because the variable $a no longer exists once this function exits. To write a counting function that does not lose the current count value, define the variable $a as static:
Example 12-5. Example of using static variables
Now, each call to the Test() function will output the value of $a and increment it by one.
Static variables also provide a way to deal with recursive functions. A recursive function is a function that calls itself. Be careful when writing recursive functions, as they may recurse indefinitely. You must ensure that there are adequate ways to terminate recursion. Consider this simple function that recursively counts to 10, using the static variable $count to determine when to stop:
Example 12-6. Static variables and recursive functions
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function Test()<🎜>
{<🎜>
static $count = 0;<🎜>
<🎜> $count++;<🎜>
echo $count;<🎜>
if ($count < 10) {<🎜>
Test ();<🎜>
}<🎜>
$count--;<🎜>
}<🎜>
?>
Note: Static variables can be declared as in the above example. Assigning it with the result of an expression in a declaration will result in a parsing error.
Example 12-7. Declare static variables
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function foo(){
static $int = 0; // correct
static $int = 1+2; // wrong (as it is an expression)
static $int = sqrt(121); // wrong (as it is an expression too)
$int++;
echo $int;
}
?>
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function foo(){
static $int = 0; // correct
static $int = 1+2; // wrong (as it is an expression)
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function test_global_ref() {
global $obj;
$obj = &new stdclass;
}
function test_global_noref() {
global $obj;
$obj = new stdclass;
}
References to global and static variables
In the first generation of the Zend engine, which drives PHP4, static and global definitions of variables are implemented in the form of references. For example, a true global variable imported with the global statement inside a function scope actually establishes a reference to the global variable. This can lead to unexpected behavior, as the following example demonstrates:
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function test_global_ref() {
global $obj;
$obj = &new stdclass;
}function test_global_noref() {
global $obj;
$obj = new stdclass;
}test_global_ref();
var_dump($obj);
test_global_noref();
var_dump($obj);
?>
Executing the above example will result in the following output:
NULLobject(stdClass)(0) {}
Similar behavior also applies to static statements. References are not stored statically:
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function &get_instance_ref() {<🎜>
static $obj;<🎜>
<🎜> echo "Static object: ";<🎜>
var_dump($obj);<🎜>
if (!isset($obj)) {<🎜>
//Assign a reference to a static variable<🎜>
$obj = &new stdclass;<🎜>
}<🎜>
$obj->property++;
Return $obj;
}
function &get_instance_noref() {
static $obj;
echo "Static object: ";
var_dump($obj);
if (!isset($obj)) {
//Assign an object to a static variable
$obj = new stdclass;
}
$obj->property++;
Return $obj;
}
$obj1 = get_instance_ref();
$still_obj1 = get_instance_ref();
echo "/n";
$obj2 = get_instance_noref();
$still_obj2 = get_instance_noref();
?>
Executing the above example will result in the following output:
Static object: NULLStatic object: NULLStatic object: NULLStatic object: object(stdClass)(1) { ["property"]=> int(1)}
The above example demonstrates that when a reference is assigned to a static variable, its value is not remembered the second time the &get_instance_ref() function is called
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