Single and double quoted

For most people, the single and doubles quotes in PHP are equal. But fundamental differences exists. Using double quotes there are more special characters: \n \r \t \\ \$ \" \[0-7] (1,3) \x [0-9A-Fa-f] (1,2) while with single quotes we only have \\ \'.
Knowing this already, we can assume that the single quotes are faster. On the other hand, we must add that within the double quotes variables may be used without concatenate, but writing it directly on the string contents.
More information can be found in the PHP manual, in String section.

Finally, let's compare the times of each, in various circumstances.
View example