Entry Level PHP Web Application Development/Our First Programs: Hello, World!/DQS-Answer
A double-quoted string (also known as a double-quoted string literal) has the purpose of including a string into a program, with variable substitution. For example, the string
"$foo on you" will appear as
on you if
$foo is not defined; if
$foo contains the value
'Poo', then the resulting string will appear as
Poo on you when output to the user’s web browser.