Issues with form validation regarding an array in $_POST and the stripos() function. Is my solution correct?

I am creating an online registration form which sends user inputs to an administrator via email.

I have used quite a few checkboxes, all being sent to an array called "programmes" within $_POST by changing checkbox input name attributes to "programmes[]".

I have also implemented the following code, from the Building a simple PHP application, to prevent form injection by spammer bots

foreach($_POST as $value) {
if (stripos($value, 'Content-Type:') !== FALSE ) {
    echo "Invalid submission";
    exit();
}

After this however, every form submission would direct to the "Invalid Submission" text on an empty page. After reading a bit about the stripos() function I think I understood that its first argument is where it searches for a string value, and the second argument is the string value it searches for.

Based on this understanding I figured it was probably the array() that was preventing the code from executing properly.

So I decided to change the above block to this:

        foreach($_POST as $value) {
            // If malicious inputs are detected, exit code.

            // Checks if value is array first to run validation on its elements.
            if (gettype($value) == "array") {
                // Checks if there are elements in the array
                if(count($value) > 0) {
                    foreach($value as $element) {
                        if (stripos($element, 'Content-Type:') !== FALSE) {
                            echo "Invalid submission";
                            exit();
                        }
                    }
                }
            }
            // If value is not an array, use the check below
            elseif (stripos($value, 'Content-Type:') !== FALSE ) {
                echo "Invalid submission";
                exit();
            }
        }

This seems to have solved the problem. Have I understood this procedure correctly or did I possibly leave some kind of vulnerability loophole?