Sometimes you have to be reminded of the basics:
“How to use a dollar-sign $ in a string?”
In single quotes a dollar sign isn’t parsed as anything. Nothing is auto parsed in single quotes in php. If you use double quotes they are automatically parsed:
echo "$var"; // this will print the value of $var;
echo '$var'; // this will print $var;
echo "\$var";// this escapes the dollar sign so it will print $var;