You can call a user-defined function in PHP using a string variable that contains the function's name.

In PHP, you can call a user-defined function using a string variable that contains the function's name. The correct option is "$function_name() or ${$function_name}()" as it represents the valid syntax for calling a function with a string variable. By using the variable with parentheses () or curly brackets {}, you can invoke the function. The other mentioned options are not valid syntax for calling a function with a string variable in PHP. For further details, refer to the PHP documentation on variable functions: http://php.net/manual/en/functions.variable-functions.php