Double quotation marks allow the parsing of variables

 
<?
$name = "john";
print "hello, $name"; 
?>

1.	Double-quoted string escape sequences
2.	Double-quoted strings
3.	If you use single quotation marks to enclose the same string, the variable is not substituted:
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

Double quotation marks allow the parsing of variables : Quotation « String « PHP