function searchQuery()
{
    if (!empty($_POST['searchInput'])) {
        $searchResults = searchForBook($_POST['searchInput'], $_POST['searchMode']);
    } else {
        // Default search if no input it provided, such as on initail page load.
        $searchResults = "SELECT listing.lenderID, textbook.ISBN, listing.image,\n      listing.condition, listing.borrowerID, listing.textID, textbook.*, user.*\n      FROM listing INNER JOIN textbook ON listing.ISBN=textbook.ISBN\n      INNER JOIN user ON listing.lenderID=user.userID ";
    }
    return $searchResults;
}
<?php

require 'controller.php';
$search_key = $_POST['search'];
$search_books = searchForBook($search_key);
print_r($search_books);