Memory management forms the hidden engine behind every programming language's runtime, silently powering the execution of programs and ensuring resources are used efficiently. In PHP, while variables, arrays, and objects seem straightforward and effortless to manipulate, beneath the surface lies a complex and finely tuned system. This system revolves around structures like zvals (which hold variable information), symbol tables (which track variable names and their associated data), object stores (which manage object lifecycles), and a sophisticated garbage collection mechanism designed to automatically reclaim memory occupied by unused objects. Together, these components work in harmony to maintain performance, prevent memory leaks, and deliver the seamless experience developers often take for granted.

In this article, we’ll dive into how PHP internally stores and manages memory, examine the workings of its garbage collection system, and briefly contrast these mechanisms with manual memory management techniques found in languages like C.

The Foundation: Understanding ZVAL in PHP

In PHP, every variable—whether a simple string, an array, or an object—is represented internally using a structure called zval (pronounced "zee-val").

Each zval encapsulates four essential components:

- Value (the actual data, such as integer, string, array, object)
- Type (indicating the data type)
- Reference Count (tracking how many variables point to this value)
- Is Reference Flag (noting whether the value participates in reference sets)

A simplified C-style definition of a zval might look like:

typedef struct _zval_struct {
    zend_value value;
    zend_uint refcount;
    zend_uchar type;
    zend_uchar is_ref;
} zval;

Every PHP variable is essentially a pointer to a zval. When multiple variables point to the same zval, the reference count increases accordingly, enabling PHP to optimize memory usage efficiently. Displaying zval information:

$a = "new string";
xdebug_debug_zval('a');

The above example will output:

a: (refcount=1, is_ref=0)='new string'

→ Check more about zval in PHP Internals Book.

Manual Memory Management: A C Perspective

Before diving deeper into PHP’s automated system, it is instructive to consider how memory management is handled manually in C language:

#include <stdlib.h>
#include <string.h>
#include <stdio.h>

int main() {
    char *str = (char *)malloc(13);
    strcpy(str, "Hello, world");
    printf("%s\n", str);

    free(str);
    return 0;
}

In this case, developers must explicitly allocate malloc and deallocate free memory. Failure to properly manage memory in C can result in leaks, dangling pointers, and security vulnerabilities.

But fortunately, PHP abstracts this burden through automatic reference counting and garbage collection, allowing developers to concentrate more on application logic. PHP organizes its variables inside symbol tables. Each scope—global, function, or method—maintains its own symbol table, essentially a hash map connecting variable names to their corresponding zvals.

1. Global Symbol Table stores variables defined in the main script.
2. Local Symbol Tables are created during function and method calls and destroyed once execution exits the scope.

Symbol tables ensure that different scopes remain isolated, allowing variables in different functions or classes to exist independently without conflict.

Object Management: Behind the new Keyword

When an object is instantiated with new, PHP internally takes several steps:

- A zval of type object is created.
- An object handle (an integer) is generated.
- The object’s actual data structure is stored inside PHP's object store — a global registry that tracks all live objects.

The variable that holds the new object, like $obj, simply points to the zval, which in turn references the object via the handle.

The object store ensures efficient object tracking, automatic destruction, and garbage collection integration once objects are no longer referenced.

Understanding PHP's Garbage Collection (GC)

While PHP automatically frees simple data when no references exist, problems arise when cyclic references are formed — objects or arrays referencing each other in a loop. To address this, PHP employs an intelligent Garbage Collector (GC) based on the Mark & Sweep algorithm.

This concept is not unique to PHP; many languages like Java, Python, and Go employ garbage collection techniques to manage complex memory cycles automatically.

The Garbage Collection Lifecycle

PHP’s garbage collection process operates through the following phases:

1. Root Buffer Collection: PHP monitors zvals with potential cyclic references and stores them temporarily in a root buffer. When the root buffer fills up (around 10,000 entries by default), the GC cycle initiates.
2. Mark Phase: The engine traverses the root buffer, marking all zvals still reachable from live variables.
3. Sweep Phase: Any zval not marked as reachable is considered garbage and is destroyed, freeing associated memory.

Garbage collection can also be triggered manually using:

gc_collect_cycles();

Or its status can be checked via:

gc_status();

The gc_status() output reveals statistics like the number of collection runs, collected zvals, and root buffer usage.

Root Buffer Saturation and Cyclic References

Certain structures, such as mutually referencing objects, can easily cause cyclic references:

class A { public $b; }
class B { public $a; }

$a = new A();
$b = new B();

$a->b = $b;
$b->a = $a;

Even after $a and $b are unset, the memory would not be immediately freed because of the circular reference.
This is where garbage collection steps in—detecting unreachable cycles and cleaning them up.

Manually forcing collection:

unset($a, $b);
gc_collect_cycles();

ensures proper memory cleanup.

Garbage Collection Beyond PHP

Garbage collection is a broader concept in computer science.
Languages such as Java (using a generational GC model), Python (using reference counting with cycle detection), and Go (using concurrent garbage collectors) incorporate sophisticated GC systems to manage memory automatically.

PHP’s garbage collector aligns with these principles, ensuring developers benefit from safe and efficient memory management without low-level intervention.

Summarized Insights

Final Thoughts

Mastering the internal mechanics of PHP memory management—from zvals to symbol tables and garbage collection—offers valuable insights into how PHP achieves high efficiency behind its user-friendly syntax.

This understanding bridges the gap between everyday PHP development and the robust, elegant engine that powers it, equipping developers to write cleaner, more efficient, and more scalable applications.

→ You may read the PHP manual for more information.