What error message did the user receive when trying a suggested modification to the code?

The error message the user received when trying a suggested modification to the code was "Fatal error: Can't use function return value in write context." This error occurs when a function's return value is used in a context where a variable is expected, such as in an if statement. To solve this issue, the user needs to assign the function's return value to a variable before using it in the if statement.

// Before modification
if (is_admin()) {
    // do something
}

// After modification
$is_admin = is_admin();
if ($is_admin) {
    // do something
}