PHP: Single Quotes versus Double Quotes

�To start, let's get some sample code together; first for single quotes:

<?php
$string = 'This is a string';
$string2 = 'This is a string that includes a ' . $string;
$string3 = 'One last string include a ' . $string2;
echo $string;
echo $string2;
echo $string3;

Next, let's do almost the exact same code using double quotes:

<?php
$string = "This is a string";
$string2 = "This is a string that includes a $string";
$string3 = "One last string include a {$string2}";
echo $string;
echo $string2;
echo $string3;

Once finished, I proceeded to run my speed comparison tool hosted on GitHub.� The results are fascinating!� The single quote example ran just under 7,400 times in 10 seconds.� The double quote example ran just under 7,900 times in 10 seconds.� A 7% speed increase by using double quotes.

At first glance, I was surprised by this result.� But, then I stopped and thought about it for a second.� If double quoted strings are already being processed by the compiler, it should deal quite well with processing the variables within them; whereas, single quoted strings would need to be evaluated and then determined that they need to be processed because I'm concatenating a string with it!

I then proceeded to adjust the examples as follows:

<?php
$string = 'This is a string';
echo $string;

Next, let's do almost the exact same code using double quotes:

<?php
$string = "This is a string";
echo $string;

These results were more of what I expected to see.�� The single quote example ran nearly 7,800 times while the double quoted example ran just under 7,300 times.� This makes much more sense, since the single quoted string requires no processing and the double quoted string needs processing (even though there is nothing to process) slows things down a bit.

Summary

Well, I think my mind has been officially changed � now I just need to change my coding style � when I need to include variables inside of variables in PHP I am going to use double quoted strings.� However, when I have more of constant based string I will continue to use single quoted strings for the exact reason that they do not get executed by the compiler.

Published on Oct 9, 2012

Tags: Optimization | php | speed comparison | Theory | PHP