What is the significance of the error message "Fatal error: Unparenthesized `a ? b : c ? d" in PHP scripts?

The error message "Fatal error: Unparenthesized `a ? b : c ? d" occurs when the ternary operator in a PHP script is not properly parenthesized. To solve this issue, you need to add parentheses to clarify the order of operations in the ternary expression.

// Incorrect ternary expression causing the error
$result = $a ? $b : $c ? $d;

// Corrected ternary expression with parentheses
$result = $a ? $b : ($c ? $d : null);