411 REFERENCE · Wikibooks
Entry Level PHP Web Application Development/Our First Programs: Hello, World!/DQS-Answer
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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.