php

the difference between unset and = null

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From a random php.net post:

If you are doing $whatever = null; then you are rewriting variable's data. You might get memory freed / shrunk faster, but it may steal CPU cycles from the code that truly needs them sooner, resulting in a longer overall execution time.

Apparently this is the undisputed truth so maybe someone would be so kind as to explain.

I mean, what, does unset magically not execute any assembly instructions whereas $whatever = null; does? The answer, as given, is about as useful as saying

$whatever = null resets the buffer and the L1 cache whereas unset clears the buffer and resets the L2 cache.

Techno mumbo jumbo doesn't constitute an answer.

Answers

An important difference between both methods is that unset($a) also removes $a from the symbol table; for example:

$a = str_repeat('hello world ', 100);
unset($a);
var_dump($a);

Outputs:

Notice: Undefined variable: a in xxx
NULL

But when $a = null is used:

$a = str_repeat('hello world ', 100);
$a = null;
var_dump($a);

Outputs:

NULL

I ran the code through a benchmark as well and found that $a = null is roughly 6% faster than its unset() counterpart. It seems that updating a symbol table entry is faster than removing it.

Addendum

The other difference (as seen in this small script) seems to be how much memory is restored after each call:

echo memory_get_usage(), PHP_EOL;
$a = str_repeat('hello world ', 100);
echo memory_get_usage(), PHP_EOL;
// EITHER unset($a); OR $a = null;
echo memory_get_usage(), PHP_EOL;

When using unset() all but 64 bytes of memory are given back, whereas $a = null; frees all but 272 bytes of memory. I don't have enough knowledge to know why there's a 208 bytes difference between both methods, but it's a difference nonetheless.

Wednesday, November 2, 2022

awmuller13-61ac8a9edf71

Here's what worked best for me when trying to script this (in case anyone else comes across this like I did):

$ pecl -d php_suffix=5.6 install <package>
$ pecl uninstall -r <package>

$ pecl -d php_suffix=7.0 install <package>
$ pecl uninstall -r <package>

$ pecl -d php_suffix=7.1 install <package>
$ pecl uninstall -r <package>

The -d php_suffix=<version> piece allows you to set config values at run time vs pre-setting them with pecl config-set. The uninstall -r bit does not actually uninstall it (from the docs):

vagrant@homestead:~$ pecl help uninstall
pecl uninstall [options] [channel/]<package> ...
Uninstalls one or more PEAR packages.  More than one package may be
specified at once.  Prefix with channel name to uninstall from a
channel not in your default channel (pecl.php.net)

Options:
  ...
  -r, --register-only
        do not remove files, only register the packages as not installed
  ...

The uninstall line is necessary otherwise installing it will remove any previously installed version, even if it was for a different PHP version (ex: Installing an extension for PHP 7.0 would remove the 5.6 version if the package was still registered as installed).

Monday, December 12, 2022

shankar_damodaran

Take an uninitialized pointer:

int* ptr;//points to any location in memory

Take a null pointer:

int* ptr = NULL;//normally points to 0x0 (0)

Both would cause undefined behaviour if dereferenced. NULL is often defined as 0.

Tuesday, September 13, 2022

matprzybyla

Like @Jack said x ne null is equal to !(x eq null). The difference between x != null and x ne null is that != checks for value equality and ne checks for reference equality.

Example:

scala> case class Foo(x: Int)
defined class Foo

scala> Foo(2) != Foo(2)
res0: Boolean = false

scala> Foo(2) ne Foo(2)
res1: Boolean = true

Sunday, October 23, 2022

blytung

Never used any of those, but they look interesting..

Take a look at Gearman as well.. more overhead in systems like these but you get other cool stuff :) Guess it depends on your needs ..

Friday, November 11, 2022

the_falvert

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