What is the difference between assignment by value and assignment by reference in PHP?

Assignment by value: When assigning the value of an expression to a variable, the value of the entire original expression is assigned to the target variable. This means that, for example, changing the value of one variable while the value of one variable is assigned to another variable will not affect the other variable.

<?php 
$a=123; $a=123; 
$b=$a; $b=&$a; 
$a=321; $a=321; 
Echo”$a,$b”;//显示”321,123” Echo”$a,$b”;//显示”321,321” 
?>

Reference assignment: The new variable simply references the original variable. Changing the new variable will affect the original variable. Using reference assignment, simply add an & symbol in front of the variable to be assigned ( Source variable)
Type trick PHP does not require (or does not support) explicit type definitions in variable definitions; the variable type is determined based on the context in which the variable is used. That is, if you assign a string value to the variable var, var becomes a string. If you assign an integer value to var, it becomes an integer.
Type casting
The allowed castings are: (int), (integer) - converted to integer type (bool), (boolean) - converted to Boolean type (float), (double), (real) - Convert to floating point type (string) - Convert to string (array) - Convert to array (object) - Convert to object Settype() for type conversion
Function Settype()

<?php 
$foo = "5bar"; // string 
$bar = true; // boolean 
settype($foo, "integer"); // $foo 现在是 5 (integer) 
settype($bar, "string"); // $bar 现在是 "1" (string) 
?>

Variable scope The scope of a variable is the context in which it is defined (that is, its effective scope). Most PHP variables have only a single scope. This single scope span also includes files introduced by include and require.
Static variable Another important feature of variable scope is static variable (static variable). Static variables only exist in the local function scope, but their values ​​are not lost when program execution leaves this scope.
Array An array in PHP is actually an ordered graph. A graph is a type that maps values ​​to keys. This type is optimized in many ways, so it can be used as a real array, or a list (vector), a hash table (an implementation of a graph), a dictionary, a set, a stack, a queue, and many more possibilities. Since you can use another PHP array as a value, you can also simulate a tree easily.
Definition array() You can use the array() language structure to create a new array. It accepts a number of comma-separated key => value parameter pairs.
array( key => value , ... )
// key can be integer or string
// value can be any value

<?php // 现在删除其中的所有单元，但保持数组本身的结构 
// 创建一个简单的数组 foreach ($array as $i => $value) { 
$array = array(1, 2, 3, 4, 5); unset($array[$i]); 
print_r($array); } 
print_r($array); 
// 添加一个单元（注意新的键名是 5，而不是你可能以为的 0） 
$array[] = 6; 
print_r($array); // 重新索引： 
$array = array_values($array); 
$array[] = 7; 
print_r($array); 
?>

unset() function allows to cancel a The key name in the array. Be aware that the array will not be reindexed.

<?PHP 
$a = array( 1 => &#39;one&#39;, 2 => &#39;two&#39;, 3 => &#39;three&#39; ); 
unset( $a[2] ); 
/* 将产生一个数组，定义为 
$a = array( 1=>&#39;one&#39;, 3=>&#39;three&#39;); 
而不是 
$a = array( 1 => &#39;one&#39;, 2 => &#39;three&#39;); 
*/ 
$b = array_values($a); 
// Now $b is array(0 => &#39;one&#39;, 1 =>&#39;three&#39;) 
?>

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