Conclusion

As you can see, in tests where variables are interpolated into the string single quotes with concatenation ('some '.$v1.' string '.$v2) is always faster than double quotes with interpolation ("some $v1 string $v"). In tests where the string is a straight literal both are about the same speed.

Using curly braces around variables in double quotes makes no discernable difference to speed in the first set of tests. In the eval() tests using curly braces is definately slower.

HEREDOC is about the same speed as double quotes when it includes variables, but is slower than both double and single quotes when it is only a literal. Don't use HEREDOC at all if you can avoid it.

sprintf() is slower than anything else when doing simple string interpolation. It's about 3x slower than single quotes whereas double quotes is about 2x slower than single quotes. Don't use sprintf() when doing simple string replacement (sorry, C programmers).

Outer braces ({$var}) are slightly faster than inner braces (${var}) when using simple variables but slower when using complicated variables (such as arrays).

When putting many variables' values into a string using double quotes rather than single quotes is faster somewhere between 20 and 50 interpolations, depending on whether you use an opcode cache and the complexity of the variables.

When putting many variables' values into a string using sprintf rather than single quotes the win is much larger and shows up before 5 interpolations when not using an opcoce cache and around 20 when using one. Of course this is only for this many inerpolations in a single string. When you only have a few interpolations single quotes are still much faster.

Of course, this test was made over 1,000,000 iterations of each test string and the differences are in seconds but the difference is up to a 50% speedup. This can be a significant performance gain with a script which has lots of string processing or on a high volume site.

Other Reasons to Use Single Quotes

Single quotes are also better to use for reasons other than speed.

• It is easier to audit code with single quotes as you can easily skip over strings with single quotes when checking for possible XSS attacks and other places where user input could affect the security or usability of your code
• Using single quotes and concatenation makes it easier to indent and seperate literal strings and variables, making it even easier to audit code
• Many syntax highlighters don't highlight variables within double quotes correcly or fully
• Using variables in double quotes limits the syntax that you can use for variables and can lead to unneccesary use of temporary variables and inadvertent bugs
• Using variables in double quotes promotes lazy array access ($arr[key] vs. $arr['key'])

Source