What is PHP garbage collection mechanism

　The whole reason for implementing a garbage collection mechanism is to save memory usage by cleaning up circularly referenced variables.

1. Reference count: php variables exist in a variable container called "zval". A zval variable container, in addition to the type and value of the variable, also includes two bytes of additional information: is_ref and refcount. is_ref is a bool value, used to identify whether this variable belongs to a reference collection, so that the PHP engine can distinguish ordinary variables from reference variables; refcount is used to represent the number of variables pointing to this zval variable container. When refcount = 0, it means that the Variables can be cleared or recycled

   <?php
   $a = array( &#39;meaning&#39; => &#39;life&#39;, &#39;number&#39; => 42 );
   $a[&#39;life&#39;] = $a[&#39;meaning&#39;];
   xdebug_debug_zval( &#39;a&#39; );
   ?>

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   The output of the above routine is as follows:

   a: (refcount=1, is_ref=0)=array (
      'meaning' => (refcount=2, is_ref=0)='life',
      'number' => (refcount=1, is_ref=0)=42,
      'life' => (refcount=2, is_ref=0)='life'
   )

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   The output of The results are as follows:

   <?php
   $a = array( &#39;one&#39; );
   $a[] =& $a;
   xdebug_debug_zval( &#39;a&#39; );
   unset($a);
   xdebug_debug_zval( &#39;a&#39; );
   ?>

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   In the above example, although there is no longer any symbol in a scope pointing to this variable container, since the array element "1" still points to the array itself, this container cannot Cleared. Because there is no other symbol pointing to it, the user has no way to clear the structure, resulting in a memory leak. PHP will clear this data structure at the end of script execution, but before PHP clears it, it will consume a lot of memory

   Recycling cycle: Can be used to deal with memory leaks caused by circular references. If the reference count of a variable container increases, it will continue to be used and of course will no longer be in the garbage; if the reference count decreases to zero, the variable container will be cleared (free). That is to say, a garbage cycle will occur only when the reference count is reduced to a non-zero value; secondly, in a garbage cycle, by checking whether the reference count is reduced by 1 (simulation), and checking which variable containers have references The number of times is zero, to find out which part is garbage

2. Turn on and off the garbage collection mechanism: In addition to modifying the configuration

   zend.enable_gc , you can also turn on and off the garbage collection mechanism by calling the
gc_enable() and
3. gc_disable() functions respectively. In addition,

   Possible roots are recorded even when the garbage collection mechanism is unavailable, so that every time a possible root is found, it is not checked whether the garbage collection mechanism is turned on, and the recording operation is faster. Call the gc_collect_cycles() function to force cycle recycling.

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