What are the fundamental PHP concepts that the forum user seems to be lacking, based on the responses provided by other users?

The forum user seems to be lacking an understanding of how to properly concatenate variables within a string in PHP. They are trying to concatenate variables using the `+` operator instead of the `.` operator. To solve this issue, the user should use the `.` operator to concatenate variables within a string in PHP.

// Incorrect way of concatenating variables using the + operator
$name = "John";
$age = 25;
$message = "Hello, my name is " + $name + " and I am " + $age + " years old.";

// Correct way of concatenating variables using the . operator
$name = "John";
$age = 25;
$message = "Hello, my name is " . $name . " and I am " . $age . " years old.";