If you use single quotation marks to enclose the same string, the variable is not substituted:

 
<?
print 'hello, $name'; // hello, $name
?>

Double-quoted strings are also parsed for escape characters.

1.	Double quotation marks allow the parsing of variables
2.	Double-quoted string escape sequences
3.	Double-quoted strings
4.	Using forward slashes on Windows
5.	Fixing quote escaping in backreference replacements
6.	It is safe to use non-escaped Windows-style filenames in your single-quoted strings
7.	Various special characters in string assignments

If you use single quotation marks to enclose the same string, the variable is not substituted: : Quotation « String « PHP