Single or double quotes in PHP programming are used to define a string. But, there are lots of differences between these two.
Single-quoted Strings: It is the easiest way to define a string. You can use it when you want the string to be exactly as it is written. All the escape sequences like \r or \n, will be output as specified instead of having any special meaning. Single-quote is usually faster in some cases. The special case is that if you to display a literal single-quote, escape it with a backslash (\) and if you want to display a backslash, you can escape it with another backslash (\\).
Below program illustrates the Single-quoted Strings:
I am a geek. It'll be interesting to know about the string. A \ is named as backslash. This is a portal for $string. This is a portal for \n geeks.
Double-quoted strings: By using Double quotes the PHP code is forced to evaluate the whole string. The main difference between double quotes and single quotes is that by using double quotes, you can include variables directly within the string. It interprets the Escape sequences. Each variable will be replaced by its value.
Below program illustrates the Double-quoted Strings:
I am a geek. It'll be interesting to know about the string. This is a simple string. The word is ABC.