Single quote
Alternatively referred to as an apostrophe a single quote is a symbol on the keyboard that looks like a small single vertical lines ( ' ) that is found on the United States QWERTY keyboard by the Enter key. In computer programming single quotes are used to contain commands or literal strings. In the below example, the variable $example would not be treated as a variable, instead it would print "Do not print $example".
print 'Do not print $example';
Also see: Back quote, Keyboard definitions, Quote