Challenge 3 — Solution

Task: Write out (as comments describing a 3-file structure) why using
require_once instead of require would prevent a "cannot redeclare
function" fatal error if both header.php and sidebar.php each try to
require the same functions.php file from within index.php.

<?php
    // ---- functions.php ----
    // <?php
    //     function formatPrice($amount) {
    //         return "£" . number_format($amount, 2);
    //     }
    // ?>

    // ---- header.php ----
    // <?php
    //     require_once 'functions.php';
    //     echo "<header>Store</header>";
    // ?>

    // ---- sidebar.php ----
    // <?php
    //     require_once 'functions.php';   // functions.php requested a second time
    //     echo "<aside>Cart total: " . formatPrice(24.5) . "</aside>";
    // ?>

    // ---- index.php ----
    // <?php
    //     require 'header.php';   // pulls in functions.php the 1st time (via header.php)
    //     require 'sidebar.php';  // pulls in functions.php the 2nd time (via sidebar.php)
    // ?>

    // Why require_once (not plain require) matters here:
    // Both header.php and sidebar.php independently try to load
    // functions.php. If each one used a plain "require 'functions.php';",
    // the function formatPrice() would be defined once when header.php
    // runs, and PHP would then try to define it AGAIN when sidebar.php
    // runs its own require - triggering a fatal
    // "Cannot redeclare formatPrice()" error, since PHP does not allow
    // the same function name to be defined twice.
    //
    // Because both files use require_once instead, PHP remembers that
    // functions.php was already loaded the first time (via header.php)
    // and simply skips loading it again the second time (via
    // sidebar.php) - formatPrice() ends up defined exactly once, and
    // both header.php and sidebar.php can still safely call it.

    // A working single-file demonstration of the underlying idea:
    function formatPrice($amount) {
        return "£" . number_format($amount, 2);
    }
    echo formatPrice(24.5);
?>


Output:
£24.50

Notes:
- The failure mode being avoided here is specific to files that DEFINE
  something (functions, classes) rather than files that only run
  top-level code - a file that just echoes HTML can safely be require'd
  twice with no error, but a file that defines a function cannot.
- require_once and include_once both track, per script run, exactly
  which files have already been loaded, and silently skip any repeat
  request for a file already on that list - regardless of which
  including file made the earlier request.
- This is exactly why a shared function/class library that might be
  pulled in from multiple different included files (as header.php and
  sidebar.php both do here) should almost always use require_once
  rather than a plain require.