Multiplication and addition are calculated wrongly


The exercise is on this page Coding Games and Programming Challenges to Code Better

"Exercise 1: Debugging
Here is a program whose objective is to calculate the total area of two rectangles whose lengths and widths are given successively by the function lireLigne() in the following order: the length then the width of the first rectangle and then, the length then the width of rectangle 2.

Unfortunately, this program contains many errors, both in the PHP code (syntax) and in the calculation.

Fixes the program so that it can work correctly"

This is what I wrote but this code is not accepted, why?
The final amount is calculated wrongly

$longueurRect1 = lireLigne();
$largeurRect1 = lireLigne();
$longueurRect2 = lireLigne();
$largeurRect2 = lireLigne();
$aireTotale = $longueurRect1 * $largeurRect1 + $longueurRect2 * $largeurRect2;
//On affiche le résultat
echo 'L\'Aire totale est égale à $aireTotale cm²';

You need to use double quote " instead of single quote ’ to replace $aireTotale with its value in the output.
see php docs here