Double quotations will evaulate included variables, while single quotations will interpret the whole string literally.

Below, the examples show a message where the $date variable is included at the end of the string.

The first uses double quotations, and $date is interpreted to be the value "December 31st, 1969."

The second uses single quotations, and $date is printed as $date.

(Double Quotations) Today's date is December 31st, 1969

(Single Quotations) Today's date is $date