Study notes on PHP variables and memory management

1. memory_get_usage function

int memory_get_usage ([ bool $real_usage = false ] )

Memory used by PHP script (excluding memory_get_usage() The memory occupied by the function itself)

The parameter $real_usage of memory_get_usage() defaults to FALSE; when set to TRUE, the memory value obtained is the memory occupied by PHP memory management.

//php分配到的内存
var_dump(memory_get_usage(true));
//php使用内内存
var_dump(memory_get_usage());

Result

int 262144
int 239368

variables, and when the space is not enough, apply for a new space.

var_dump(memory_get_usage());
 
$a = "Hello World";
var_dump(memory_get_usage());
 
$b = $a;
var_dump(memory_get_usage());
 
unset($b);
var_dump(memory_get_usage());
 
unset($a);
var_dump(memory_get_usage());

int 239912
int 240000
int 239912
int 239776

##239912-239776=136

240000-239912=88

The memory used by $a and $b assignments is different. The reason is that $b assignment does not allocate memory. These 88 bytes are occupied by the output function.

In addition, destroying $b does not affect the memory used by $a. This is the reason for memory management reference counting.

You can clearly understand the relationship between variable assignment and memory by changing the example

var_dump(memory_get_usage());
 
$a = "Hello World";
var_dump(memory_get_usage());
 
$b = "Hello World";
var_dump(memory_get_usage());
 
unset($b);
var_dump(memory_get_usage());
 
unset($a);
var_dump(memory_get_usage());

Result

int 239816

int 240088
int 239952
int 239816


239952-239816=136

240088-239952=136

In addition, the memory occupied by reference assignment is similar to that of direct assignment, which also involves the reference counting rules of memory management

3. PHP garbage collection mechanism, this part is not understood deeply enough, simply record a function

xdebug_debug_zval needs to install the xdebug extension

$a = "Hello World";
xdebug_debug_zval('a');

Output

a:

(refcount=1, is_ref=0),

'Hello World' (length=11)
refcount points to The number of variables of this value

is_ref represents whether there is an address reference

Type

Value

When assigning a variable: is_ref is false and refcount is 1

Manual address

Basic knowledge of reference counting

http://php.net/manual/zh/features.gc.refcounting-basics.php

Each

exists in a variable container called "zval". A zval variable container contains, in addition to the type and value of the variable, two bytes of additional information. The first one is "is_ref", which is a bool value used to identify whether this variable belongs to the reference set. Through this byte, the PHP engine can distinguish ordinary variables from reference variables. Since PHP allows users to use custom references by using &, there is also an internal reference counting mechanism in the zval variable container to optimize memory usage. The second extra byte is "refcount", which is used to indicate the number of variables (also called symbols) pointing to this zval variable container. All symbols exist in a symbol table, where each symbol has a scope (scope), the main script (for example: the script requested through the browser) and each function or method also have a scope.

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