


How does PHP's garbage collection mechanism work, particularly with circular references?
Jun 04, 2025 pm 03:53 PMPHP has dealt with memory leaks caused by circular references starting from version 5.3 by building possible root object graphs and periodically analyzing them. The specific steps are: 1. Use reference count to track variables; 2. Build possible root object graphs during execution; 3. Periodically or manually trigger analysis and free recycled reference memory. Automatic triggering is based on internal heuristic algorithms, or you can call gc_collect_cycles() manually or run at the end of the script. For long-running scripts, it is recommended to manually trigger GC to reduce memory usage, and pay attention to rationally designing object reference structures and using memory monitoring tools to assist in optimization.
PHP's garbage collection system handles memory management automatically, but one of its trickier aspects involves circular references — when two or more objects reference each other, potentially causing memory leaks. Let's break down how PHP deals with this.
What are circular references and why do they matter?
A circular reference occurs when two or more variables or objects refer to each other directly or indirectly. For example:
$a = new stdClass(); $b = new stdClass(); $a->b = $b; $b->a = $a;
In this case, $a
and $b
form a cycle. If both go out of scope but still reference each other, a naive garbage collector might miss them because each has a reference count of at least 1.
Before PHP 5.3, the default reference counting mechanism could't detect these cycles, which means such objects wouldn't be cleaned up, leading to memory leaks.
How does PHP handle garbage collection?
Starting from PHP 5.3, an improved garbage collection mechanism was introduced that specifically targets these circular references.
Here's how it works in short:
- PHP uses reference counting for most variable tracking. As long as something points to a variable, it stays in memory.
- But for circular references, PHP builds a graph of possible root candidates (called "possible roots") during normal execution.
- Periodically, or when triggered manually using
gc_collect_cycles()
, PHP analyzes these candidates to detect actual cycles and frees the memory accordingly.
This means even if two objects point to each other and no one else does, PHP will recognize the cycle and clean them up.
When does garbage collection run?
Garbage collection doesn't run all the time — that would be essential. Instead, PHP triggers it under certain conditions:
- Automatically based on internal heuristics (eg, after a certain number of allocations).
- Manually by calling
gc_collect_cycles()
. - At the end of a script's execution.
You can also control some behaviors via php.ini
settings like zend.enable_gc
, though it's enabled by default.
If you're dealing with large data structures or long-running scripts (like daemons or command-line tools), explicitly calling gc_collect_cycles()
may help keep memory usage lower.
Practical tips for managing memory in PHP
- Don't worry too much about small scripts — PHP's GC usually handles things well.
- In long-running processes, consider manually triggering GC after heavy operations.
- Be cautious when building object graphs that link to each other; while PHP handles cycles now, it's still good practice to break references when done.
- Use tools like
memory_get_usage()
to monitor memory behavior during development or debugging.
For example, if you're processing thousands of objects in a loop, resetting variables or unsetting references between iterations can help the GC identify unused memory faster.
That's basically how PHP's garbage collection works with circular references — not perfect, but solid enough for most use cases. It's easy to overlook, but knowing how it behaves helps avoid subtle memory issues.
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