Global variable or pass variable in PHP? (performance)

Question

If I have the option of using a global variable or passing a variable, which option is better in terms of speed and memory usage?

// global variable
function func(){
  global $var;
  echo $var;
}

$var='string';
func();


// passing variable
function func($var){
  echo $var;
}

$var='string';
func($var);

I don't think it will make much of a difference, and readability of code is important. Even if passing variables is slightly slower (and I don't know if it is) you should still generally avoid global variables. — halfer
– halfer, Commented Feb 25, 2014 at 0:07
With such little code, I would assume that there's no measurable differenc — Nico Haase
– Nico Haase, Commented Jan 13 at 10:51

Kevin Vaughan · Accepted Answer · 2014-02-25 00:55:03Z

6

TLDR: Pass by parameter = $GLOBALS element >>> global $var

When in doubt, test! The below results show that:

Passing a 1Mb string by parameter is vastly more performant than ref by global $var
Passing a 1Mb string by parameter is approximately equally performant as using $GLOBALS['var']
Using global $var in the below manner seems to mess with GC reference counts on the memory and is ridiculously slow. Apparently, don't use global $var for cases like this made up one.

Results (see code further below):
Time is seconds elapsed, memory is memory potentially leaked.

$ php -e test.php
Pass value by parameter
Time: 0.20166087150574s
Memory: 0

Global var reference
Time: 70.613216876984s
Memory: 1048576

GLOBALS array reference
Time: 0.22573900222778s
Memory: 0

Test Code:

<?php

$baseVar = str_repeat('x', 1000000);
$GLOBALS['myVar'] = $baseVar;

function testfunc_param($paramVar) {
    $localVar = $paramVar;
    return $localVar;
}

function testfunc_global() {
    global $myVar;
    $localVar = $myVar;
    return $localVar;
}

function testfunc_globalsarray() {
    $localVar = $GLOBALS['myVar'];
    return $localVar;
}


// Testing passing value by parameter
memory_get_usage(); // in case this procs garbage collection
$memoryStart = memory_get_usage(true);
$timeStart = microtime(true);
for ($i = 0; $i < 1000000; $i++) {
    testfunc_param($baseVar);
}
$timeEnd = microtime(true);
$memoryEnd = memory_get_usage(true);
print "Pass value by parameter\nTime: ".($timeEnd - $timeStart)."s\nMemory: ".($memoryEnd-$memoryStart)."\n\n";


// Testing reference to global variable
memory_get_usage(); // in case this procs garbage collection
$memoryStart = memory_get_usage(true);
$timeStart = microtime(true);
for ($i = 0; $i < 1000000; $i++) {
    testfunc_global();
}
$timeEnd = microtime(true);
$memoryEnd = memory_get_usage(true);
print "Global var reference\nTime: ".($timeEnd - $timeStart)."s\nMemory: ".($memoryEnd-$memoryStart)."\n\n";


// Testing reference to global variable via $GLOBALS
memory_get_usage(); // in case this procs garbage collection
$memoryStart = memory_get_usage(true);
$timeStart = microtime(true);
for ($i = 0; $i < 1000000; $i++) {
    testfunc_globalsarray();
}
$timeEnd = microtime(true);
$memoryEnd = memory_get_usage(true);
print "GLOBALS array reference\nTime: ".($timeEnd - $timeStart)."s\nMemory: ".($memoryEnd-$memoryStart)."\n\n";

answered Feb 25, 2014 at 0:55

Kevin Vaughan

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4 Comments

Don Viegues Over a year ago

I have tested your code, and for me, difference is 0.005 in most cases. Using PHP7.0. Thanks.

Kevin Vaughan Over a year ago

Indeed. These results were from 2014. See @magallanes answer above for updates using modern php versions.

Thomas Ingham Over a year ago

@KevinVaughan Boop. .

Elect2 Over a year ago

Now they are same (year 2023, PHP 8.2). I tested the script. The three functions are all around 0.02s, memory are all 0.

Guntram Blohm · Accepted Answer · 2014-02-25 00:07:29Z

3

The global variable will probably be faster, but not in a way that it's detectable unless you microbenchmark it. So base your decisions on which code is more readable/maintainable (which will be passing the variable in almost all cases), not some speed advantage you'll normally never notice.

answered Feb 25, 2014 at 0:07

Guntram Blohm

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Comments

question_please · Accepted Answer · 2025-01-13 10:49:39Z

Tl/dr: define const=const > parameter > global > $GLOBAL

However, the difference is really marginal, for 10 millions of operations, the difference is less than a second.

I ran the example of @Kevin Vaughan (I ran it 10'000'000 instead of 1'000'000) Windows 10 64bits, php 7.2 64bits

    Pass value by parameter
Time: 0.62202191352844s
Memory: 0

Global var reference
Time: 0.70083403587341s
Memory: 0

GLOBALS array reference
Time: 0.84828305244446s
Memory: 0

GLOBALS array reference2
Time: 0.80545091629028s
Memory: 0

GLOBALS array const define
Time: 0.57029700279236s
Memory: 0

GLOBALS array const
Time: 0.57260584831238s
Memory: 0

The memory leak is gone, however, the difference of performance is really small. Yet, passing the values by parameter is still fast.

Updated code with new functions.

<?php
echo "<pre>";
$baseVar = str_repeat('x', 1000000);
$GLOBALS['myVar'] = $baseVar;
define('BASEVAR',$baseVar);

const BASEVAR2=BASEVAR;

function testfunc_param($paramVar) {
    $localVar = $paramVar;
    return $localVar;
}

function testfunc_global() {
    global $myVar;
    $localVar = $myVar;
    return $localVar;
}

function testfunc_globalsarray() {
    $localVar = $GLOBALS['myVar'];
    return $localVar;
}

function testfunc_globalsarray2() {
    return $GLOBALS['myVar'];
}

function testfunc_const() {
    return BASEVAR;
}

function testfunc_const2() {
    return BASEVAR2;
}

// Testing passing value by parameter
memory_get_usage(); // in case this procs garbage collection
$memoryStart = memory_get_usage(true);
$timeStart = microtime(true);
for ($i = 0; $i < 10000000; $i++) {
    testfunc_param($baseVar);
}
$timeEnd = microtime(true);
$memoryEnd = memory_get_usage(true);
print "Pass value by parameter\nTime: ".($timeEnd - $timeStart)."s\nMemory: ".($memoryEnd-$memoryStart)."\n\n";


// Testing reference to global variable
memory_get_usage(); // in case this procs garbage collection
$memoryStart = memory_get_usage(true);
$timeStart = microtime(true);
for ($i = 0; $i < 10000000; $i++) {
    testfunc_global();
}
$timeEnd = microtime(true);
$memoryEnd = memory_get_usage(true);
print "Global var reference\nTime: ".($timeEnd - $timeStart)."s\nMemory: ".($memoryEnd-$memoryStart)."\n\n";


// Testing reference to global variable via $GLOBALS
memory_get_usage(); // in case this procs garbage collection
$memoryStart = memory_get_usage(true);
$timeStart = microtime(true);
for ($i = 0; $i < 10000000; $i++) {
    testfunc_globalsarray();
}
$timeEnd = microtime(true);
$memoryEnd = memory_get_usage(true);
print "GLOBALS array reference\nTime: ".($timeEnd - $timeStart)."s\nMemory: ".($memoryEnd-$memoryStart)."\n\n";

// Testing reference to global variable via $GLOBALS
memory_get_usage(); // in case this procs garbage collection
$memoryStart = memory_get_usage(true);
$timeStart = microtime(true);
for ($i = 0; $i < 10000000; $i++) {
    testfunc_globalsarray2();
}
$timeEnd = microtime(true);
$memoryEnd = memory_get_usage(true);
print "GLOBALS array reference2\nTime: ".($timeEnd - $timeStart)."s\nMemory: ".($memoryEnd-$memoryStart)."\n\n";

// Testing reference to global variable via $GLOBALS
memory_get_usage(); // in case this procs garbage collection
$memoryStart = memory_get_usage(true);
$timeStart = microtime(true);
for ($i = 0; $i < 10000000; $i++) {
    testfunc_const();
}
$timeEnd = microtime(true);
$memoryEnd = memory_get_usage(true);
print "GLOBALS array const define\nTime: ".($timeEnd - $timeStart)."s\nMemory: ".($memoryEnd-$memoryStart)."\n\n";

// Testing reference to global variable via $GLOBALS
memory_get_usage(); // in case this procs garbage collection
$memoryStart = memory_get_usage(true);
$timeStart = microtime(true);
for ($i = 0; $i < 10000000; $i++) {
    testfunc_const2();
}
$timeEnd = microtime(true);
$memoryEnd = memory_get_usage(true);
print "GLOBALS array const\nTime: ".($timeEnd - $timeStart)."s\nMemory: ".($memoryEnd-$memoryStart)."\n\n";


echo "</pre>";

Korvin Szanto · Accepted Answer · 2014-02-25 00:24:17Z

2

In PHP, global $var is essentially running $var = &$_GLOBALS['var'] behind the scenes, so you'd be looking at a search time of O(n). The cost in running the function with the variable passed in is so little that it's not worth optimizing against.

Generally, you should find a balance between what is functionally correct, what is easy to read / test, and what is fast. In this case, the speed difference is negligible, while the cost to overall readability and testability is pretty severe.

edited Feb 25, 2014 at 0:24

answered Feb 25, 2014 at 0:18

Korvin Szanto

4,5194 gold badges22 silver badges49 bronze badges

Comments

Gonz · Accepted Answer · 2014-02-25 00:14:32Z

1

I don't think you would see any performance difference. But you should be aware it's not a good idea to use a global variable just for doing it. Eventually it would be a problem, for reading the code, developing and testing.

answered Feb 25, 2014 at 0:14

Gonz

1,20913 silver badges26 bronze badges

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