again, this is the similar question with, "how to distinguish single quote (') and double quote (") in a string?" i'll try to give a brief explanation on it.
1. single quotes (') are used to enclose string which won't be parsed by php.
eg:
2. double quotes (") are used to enclose string which will be parsed by php.
eg:
When to use single quotes:
#if the string doesn't contain php vars
#if you care about micro/milliseconds spent in parsing a string
When to use double quotes:
#if the string contains php vars
#if you don't care about micro/milliseconds spent in parsing a string
Quotes in an array
When to use backslash (\) in a string / array:
if you want to escape special character or there is a quote in a parsed string.
Good luck with your project..
