PHP Garbage Collection and Memory Management

PHP reference counting reclaims most values; cyclic references can require the cycle collector.

What It Does and When to Use It

A cyclic pair survives reference counting until cycle collection. Current allocations answer how much PHP holds now; peak allocations expose workload spikes that can exceed the process limit.

Requirements, Inputs and Expected Behavior

Confirm the runtime, extension, environment variables, and permissions before running the examples in production. gc_collect_cycles() does not guarantee return of all allocated memory to the OS.

Example 1: Basic Operation

A cyclic pair survives reference counting until cycle collection.

<?php
gc_enable();
$a=new stdClass(); $b=new stdClass(); $a->peer=$b; $b->peer=$a;
unset($a,$b); echo gc_collect_cycles(), ' cycles collected';

Example 2: Behavior and Validation

Inspect GC activity rather than assuming every object needs a GC pass.

<?php
$status=gc_status();
foreach (['runs','collected','threshold'] as $key) echo $key, ': ', $status[$key]??'n/a', PHP_EOL;

Practical Example: Application Use

Distinguish cycle collection from releasing allocator memory caches.

<?php
echo 'Before: ',memory_get_usage(true),PHP_EOL;
gc_collect_cycles(); gc_mem_caches();
echo 'After: ',memory_get_usage(true),PHP_EOL;

Important Behavior and Edge Cases

The second example highlights: Inspect GC activity rather than assuming every object needs a GC pass. The practical example adds: Distinguish cycle collection from releasing allocator memory caches.

Comparison with Related Approaches

Common Beginner Mistakes

Key Takeaways

• PHP reference counting reclaims most values; cyclic references can require the cycle collector.
• Distinguish cycle collection from releasing allocator memory caches.
• Measure the representative workload before changing memory_limit or forcing garbage collection.