Cannot use isset() on the result of an expression (you can use “null !== expression” instead)

The error “cannot use isset() on the result of an expression (you can use “null !== expression” instead)” occurs when using the isset() function on the result of an expression, such as a variable or a function call. The isset() function is used to check if a variable exists and is not null.

Instead of using isset() directly on an expression, you can use the null !== expression comparison to achieve the desired result.

Here is an example to help illustrate the concept:

$variable = null;
if (null !== $variable) {
    echo "Variable is set and not null";
} else {
    echo "Variable is either not set or is null";
}

In this example, we first initialize the $variable with a value of null. We then use the null !== $variable comparison to check if the variable is set and not null. If the condition evaluates to true, the message “Variable is set and not null” is displayed. Otherwise, the message “Variable is either not set or is null” is displayed.

By using the null !== expression instead of isset(), we ensure that the variable is both set and not null, as intended.