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+In `groupsConfig.php`, usually you specify source file paths using strings like `/path/to/file.js` or `//js/file1.js` (Minify expands this to `"{$_SERVER['DOCUMENT_ROOT']}/js/file1.js"` ).
+
+Instead of a string, you may substitute an instance of class `Minify_Source`. This allows you to customize how minification is applied, and/or pull content from a non-file location (e.g. a URL).
+
+### Example: filepath
+
+In the `$spec` array, set the key `filepath` to produce a source based on a file path:
+
+```php
+$src1 = new Minify_Source(array(
+    'filepath' => '//js/file1.js',
+));
+$src2 = new Minify_Source(array(
+    'filepath' => '//js/file2.js',
+));
+
+return [
+    'js' => [$src1, $src2]
+];
+```
+
+Note the above is functionally identical to:
+```php
+return [
+    'js' => ['//js/file1.js', '//js/file2.js'],
+];
+```
+
+### Example: Specify a different minifier or none at all
+
+To change minifier, set `minifier` to a [callback](http://php.net/manual/en/language.pseudo-types.php)`*` or the empty string (for none):
+
+**`*`Prepare for `groupsConfig.php` to be executed more than once.** (This is likely if you're using the functions in `/min/utils.php`.) In practice this just means making sure functions are conditionally defined if they don't already exist, etc.
+
+```php
+$src1 = new Minify_Source(array(
+    'filepath' => '//js/file1.js',
+    'minifier' => 'myJsMinifier',
+));
+$src2 = new Minify_Source(array(
+    'filepath' => '//js/file2.js',
+    'minifier' => 'Minify::nullMinifier', // don't compress
+));
+```
+In the above, `JmyJsMinifier()` is only called when the contents of `$src1` is needed.
+
+**`*`Do _not_ use `create_function()` or anonymous functions for the minifier.** The internal names of these function tend to vary, causing endless cache misses, killing performance and filling cache storage up.
+
+## Non-File Sources
+
+You're not limited to flat js/css files, but without `filepath`, the `$spec` array must contain these keys:
+
+  * **`id `** a unique string id for this source. (e.g. `'my source'`)
+  * **`getContentFunc `** a [callback](http://php.net/manual/en/language.pseudo-types.php) that returns the content. The function is only called when the cache is rebuilt.
+  * **`contentType `** `Minify::TYPE_JS` or `Minify::TYPE_CSS`
+  * **`lastModified `** a timestamp indicating when the content last changed. (If you can't determine this quickly, you can "fake" it using a step function, causing the cache to be periodically rebuilt.)
+
+### Example: Content from a URL
+
+Here we want to fetch javascript from a URL. We don't know when it will change, so we use a stepping expression to re-fetch it every midnight:
+```php
+if (! function_exists('src1_fetch')) {
+    function src1_fetch() {
+        return file_get_contents('http://example.org/javascript.php');
+    }
+}
+$src1 = new Minify_Source([
+    'id' => 'source1',
+    'getContentFunc' => 'src1_fetch',
+    'contentType' => Minify::TYPE_JS,    
+    'lastModified' => ($_SERVER['REQUEST_TIME'] - $_SERVER['REQUEST_TIME'] % 86400),
+]);
+```
+
+If you know that the URL content only depends on a few local files, you can use the maximum of their `mtime`s as the `lastModified` key:
+```php
+$src1 = new Minify_Source([
+    'id' => 'source1',
+    'getContentFunc' => 'src1_fetch',
+    'contentType' => Minify::TYPE_JS,
+    'lastModified' => max(
+        filemtime('/path/to/javascript.php')
+        ,filemtime('/path/to/javascript_input.css')
+    ),
+]);
+```
+
+## Performance Considerations
+
+Be aware that all the code you put in `groupsConfig.php` will be evaluated upon every request like `/min/g=...`, so make it as light as possible.
+
+If you wish to keep `groupsConfig.php` "clean", you can alternately create a separate PHP script that manually sets up sources, caching, options, and calls Minify::serve().
+
+```php
+// myServer.php
+/**
+ * This script implements a Minify server for a single set of sources.
+ * If you don't want '.php' in the URL, use mod_rewrite...
+ */
+
+require __DIR__ . '/vendor/autoload.php';
+
+// setup Minify
+$cache = new Minify_Cache_File();
+$minify = new Minify($cache);
+$env = new Minify_Env();
+$sourceFactory = new Minify_Source_Factory($env, [], $cache);
+$controller = new Minify_Controller_Files($env, $sourceFactory);
+
+function src1_fetch() {
+    return file_get_contents('http://example.org/javascript.php');
+}
+
+// setup sources
+$sources = [];
+$sources[] = new Minify_Source([
+    'id' => 'source1',
+    'getContentFunc' => 'src1_fetch',
+    'contentType' => Minify::TYPE_JS,
+    'lastModified' => max(
+        filemtime('/path/to/javascript.php'),
+        filemtime('/path/to/javascript_input.js')
+    ),
+]);
+$sources[] = '//file2.js';
+$sources[] = '//file3.js';
+
+// setup serve and controller options
+$options = [
+    'files' => $sources,
+    'maxAge' => 86400,
+];
+
+// handle request
+$minify->serve($controller, $options);
+```