What is PHP memory management mechanism and garbage collection mechanism? How to understand it?

PHP memory management mechanism

var_dump(memory_get_usage());   //获取内存
$a = "laruence";                //定义一个变量
var_dump(memory_get_usage());   //定义变量之后获取内存
unset($a);                      //删除该变量
var_dump(memory_get_usage());   //删除变量后获取内存

Output result:

int 262144
int 262144
int 262144

It can be seen from the above that PHP’s memory management mechanism is: pre-given Create a space to store variables. When the space is not enough, apply for a new space.

1. Store variable names and have symbol tables.

2. Variable values ​​are stored in memory space.

3. When deleting a variable, the space where the variable value is stored will be released, but the symbol table where the variable name is located will not be reduced.

var_dump(memory_get_usage());  //获取内存
//定义100个变量
for($i=0;$i<100;$i++)
{
    $a = "test".$i;
    $$a = "hello";
}
//获取定义100个变量之后的内存
var_dump(memory_get_usage());
//定义100个变量并删除
for($i=0;$i<100;$i++)
{
    $a = "test".$i;
    unset($$a);
}
//获取删除之后的内存
var_dump(memory_get_usage());

Output result:

int 240040
int 257680
int 240856

As can be seen from the above, although the memory becomes smaller after deletion, it is still larger than before the variable was defined. This is because Although the value of the variable is deleted, the variable name is not deleted.

php garbage collection mechanism

PHP variable storage is stored in a zval container

1. Type 2. Value 3. is_ref represents whether there is an address reference 4. refcount Number of variables pointing to the value

1. When assigning values ​​to variables: is_ref is false and refcount is 1

$a = 1;
xdebug_debug_zval('a');echo PHP_EOL;

2. Assign the value of variable a to Variable b, variable b will not store the value in the memory immediately, but first points to the value of variable a until there is any operation on variable a

$b = $a;
xdebug_debug_zval('a');echo PHP_EOL;

$c = &$a;
xdebug_debug_zval('a');echo PHP_EOL;
xdebug_debug_zval('b');echo PHP_EOL;

Garbage collection:

PHP practical video tutorial

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