PHP gc_mem_caches() Function: Usage and Examples

Asks PHP's allocator to release eligible free memory pages back to the system.

What It Does and When to Use It

Read the number of bytes returned to the allocator or OS. APCu stores local application data, Redis is external and can be shared among hosts, and OPcache stores compiled code.

Requirements, Inputs and Expected Behavior

Confirm the runtime, extension, environment variables, and permissions before running the examples in production. It returns the number of bytes freed from the allocator, not live object memory.

Example 1: Basic Operation

Read the number of bytes returned to the allocator or OS.

<?php
$freed = gc_mem_caches();
 echo "Allocator memory released: {$freed} bytes";

Example 2: Behavior and Validation

Release references before requesting allocator cache cleanup.

<?php
$values = array_fill(0, 10_000, str_repeat('a', 40));
 unset($values);
 gc_collect_cycles();
 echo gc_mem_caches();

Practical Example: Application Use

Compare allocated memory before and after; the values are not a guaranteed reduction.

<?php
$current = memory_get_usage(true);
 $freed = gc_mem_caches();
 echo 'Before: ', $current, '; freed: ', $freed,
      '; after: ', memory_get_usage(true);

Important Behavior and Edge Cases

The second example highlights: Release references before requesting allocator cache cleanup. The practical example adds: Compare allocated memory before and after; the values are not a guaranteed reduction.

Comparison with Related Approaches

Common Beginner Mistakes

Key Takeaways

• Asks PHP's allocator to release eligible free memory pages back to the system.
• Compare allocated memory before and after; the values are not a guaranteed reduction.
• Define TTL, invalidation, and concurrency behavior separately from the cache storage API.