What common mistake is the user making in their PHP code when trying to switch a boolean value?

The common mistake the user is making is using a single equal sign (=) instead of a double equal sign (==) when trying to switch a boolean value. In PHP, a single equal sign is used for variable assignment, while a double equal sign is used for comparison. To correctly switch a boolean value, the user should use the not operator (!) in front of the variable to toggle its value.

// Incorrect way to switch a boolean value
$flag = true;
$flag = false; // This will always set the value to false

// Correct way to switch a boolean value
$flag = true;
$flag = !$flag; // This will toggle the value between true and false