Challenge 3 — Solution

Task: Write a function getCachedOrCompute(string $cacheFile, int
$ttlSeconds, callable $computeFn) that generalises the chapter's
file-cache example — if a fresh cache file exists, return its decoded
contents; otherwise call $computeFn(), cache its result, and return
it. Show it being used to cache the result of an expensive function
getExpensiveReport().

<?php
    function getCachedOrCompute(string $cacheFile, int $ttlSeconds, callable $computeFn) {
        if (file_exists($cacheFile) && (time() - filemtime($cacheFile)) < $ttlSeconds) {
            return json_decode(file_get_contents($cacheFile), true);
        }

        $result = $computeFn();
        file_put_contents($cacheFile, json_encode($result));
        return $result;
    }

    function getExpensiveReport(): array {
        // Simulating an expensive operation (a real version might run
        // several slow database aggregation queries here)
        sleep(3);
        return ['total_sales' => 45230, 'top_product' => 'Widget Pro'];
    }

    $report = getCachedOrCompute('/tmp/expensive_report_cache.json', 300, 'getExpensiveReport');

    print_r($report);
?>


Output (first call, no cache file exists yet):
(a real 3-second delay while getExpensiveReport() actually runs)
Array
(
    [total_sales] => 45230
    [top_product] => Widget Pro
)

Output (a second call within 300 seconds of the first):
(returns instantly — no 3-second delay — since the cache file is
still fresh)
Array
(
    [total_sales] => 45230
    [top_product] => Widget Pro
)

Notes:
- getCachedOrCompute() generalises the chapter's own getPopularPosts()
  example by accepting the compute logic itself as a callable
  parameter, rather than hardcoding one specific database query inside
  the caching function - the same underlying pattern now works for any
  expensive operation, not just this one.
- 'getExpensiveReport' is passed as a string here, which PHP recognizes
  as a callable function name and invokes correctly via $computeFn() -
  an anonymous function or a [$object, 'method'] array would work
  identically, since callable accepts any of PHP's several callable
  shapes.
- The caching logic itself - checking file_exists() and comparing
  filemtime() against the TTL - is copied directly from the chapter's
  own getPopularPosts() example, just decoupled from any one specific
  piece of data or computation.