PHP Memory management and arrays

Question

Do I need to worry about memory leaks with PHP? In particular, I have the following code that is being called from a browser. When the call finishes, is everything cleaned up properly, or, do I need to clear the memory created by the first array that was created?

class SomeClass
{
  var $someArray = array();

  function someMethod()
  {
     $this->someArray[1] = "Some Value 1";
     $this->someArray[2] = "Some Value 2";
     $this->someArray[3] = "Some Value 3";
     $this->someArray = array();
     $this->someArray[1] = "Some other Value";
     $this->someArray[2] = "Some other Value";
     $this->someArray[3] = "Some other Value";
  }
}

someMethod();

Thanks, Scott

Every script invocation starts a new PHP process. All memory is lost between those calls. — mario
– mario, Commented Jun 22, 2012 at 1:10

alex · Accepted Answer · 2012-06-22 01:33:16Z

3

Do I need to worry about memory leaks with PHP?

It's possible to have a cyclic reference in PHP where the refcount of the zval never drops to 0. This will cause a memory leak (GC won't clean up objects that have a reference to them). This has been fixed in >= PHP 5.3.

In particular, I have the following code that is being called from a browser. When the call finishes, is everything cleaned up properly, or, do I need to clear the memory created by the first array that was created?

PHP scripts have a request lifecycle (run application, return response, close application), so it shouldn't be a worry. All memory used by your application should be marked as free'd when your application finishes, ready to be overwritten on the next request.

edited Jun 22, 2012 at 1:33

answered Jun 22, 2012 at 1:13

alex

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2 Comments

Corbin Over a year ago

I believe that as of PHP 5.3 the cyclic reference problem has been fixed.

alex Over a year ago

@Corbin You're right, just found it mentioned in some slides. I'll make an edit.

Corbin · Accepted Answer · 2012-06-22 01:09:47Z

1

If you're super paranoid, you can always unset things, however, PHP is a garbage collected language meaning that unless there is a bug in the core or in an extension, there is never going to be a memory leak.

More information

On a side note, you should use the newer PHP 5 OOP syntax. And, someMethod would be an error. It would need to be $obj->someMethod() where $obj is an instance of the class.

answered Jun 22, 2012 at 1:09

Corbin

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Emil Vikström · Accepted Answer · 2012-06-22 02:10:36Z

1

There actually do exist memory problems if you run mod_php through Apache with the mpm_prefork behavior. The problem is that memory consumed by PHP is not released back to the operating system. The same Apache process can reuse the memory for subsequent requests, but it can't be used by other programs (not even other Apache processes).

One solution is to restart the processes from time to time, for example by setting the MaxRequestsPerChild setting to something rather low (100 or so, maybe lower for lightly loaded servers). The best solution is to not use mod_php at all but instead run PHP through FastCGI.

This is a sysadmin issue though, not a programmer issue.

edited Jun 22, 2012 at 2:10

answered Jun 22, 2012 at 1:59

Emil Vikström

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