Example $str 1 = "I am 16 years old. "; $str 2 = 'I am 16 years old. '; echo "I am 16 years old. <br/>"; echo 'I am 16 years old. <br/>'; echo $str 1. "<br/>"; $age = 18; $str 1 = "I am $age years old. "; $str 2 = 'I am $age years old. '; echo "I am $age years old. ". "<br/>"; // I am 18 years old. echo ' I am $age years old. '. "<br/>"; // I am $age years old.