Fatal error: cannot use isset() on the result of an expression (you can use “null !== expression” instead)

When encountering the error “fatal error: cannot use isset() on the result of an expression (you can use ‘null !== expression’ instead)”, it means that the isset() function is being used on an expression that is not a variable or cannot be evaluated directly.

In PHP, isset() is usually used to check if a variable is set and is not null. It returns true if the variable exists and has a value other than null, and false otherwise.

To fix this error, you can replace the usage of isset() with “null !== expression”. This will ensure that the expression is evaluated first, and then compare its value to null.

Let’s look at an example:

    
      $value = getValue(); // a function that may or may not return a value
      
      if (isset($value)) {
          echo "Value is set.";
      } else {
          echo "Value is not set.";
      }
    
  

In the above example, if the getValue() function returns a value, the isset() function will work without any issues. However, if the function returns an expression instead, such as a ternary operator or a method call, the error “fatal error: cannot use isset() on the result of an expression” may occur.

To solve this, you can modify the code as follows:

    
      $value = getValue(); // a function that may or may not return a value
      
      if (null !== $value) {
          echo "Value is set.";
      } else {
          echo "Value is not set.";
      }
    
  

By using “null !== expression”, we can evaluate the expression and then compare it to null, avoiding the error. This is a safe and recommended way to replace the usage of isset() when dealing with complex expressions.