Issue 57 work. Min app sets "zlib.output_compression" to "0" in case it is "1". Builder doesn't encode is it senses this option set, and runs AJAX checks for the status of it and to check for double-encoded output.

min/builder/index.php

@@ -1,5 +1,8 @@
 <?php 
 
+// check for auto-encoding
+$encodeOutput = ! ini_get('zlib.output_compression');
+
 require dirname(__FILE__) . '/../config.php';
 
 if (! $min_enableBuilder) {
@@ -28,6 +31,7 @@ h1 {margin-top:0;}
 #groupConfig {font-family:monospace;}
 b {color:#c00}
 .topNote {background: #ff9; display:inline-block; padding:.5em .6em; margin:0 0 1em;}
+.topWarning {background:#c00; color:#fff; padding:.5em .6em; margin:0 0 1em;}
     </style>
 </head>
 
@@ -110,6 +114,35 @@ source</a></small></p>
 <script type="text/javascript" 
 src="http://ajax.googleapis.com/ajax/libs/jquery/1.2.6/jquery.min.js"></script>
 
+<script type="text/javascript">
+$(function () {
+    // detection of double output encoding
+    var msg = '<p class=topWarning><strong>Warning:</strong> ';
+    var url = 'ocCheck.php?' + (new Date()).getTime();
+    $.get(url, function (ocStatus) {
+        $.get(url + '&hello=1', function (ocHello) {
+            if (ocHello != 'World!') {
+                msg += 'It appears output is being automatically compressed, interfering ' 
+                     + ' with Minify\'s own compression. ';
+                if (ocStatus == '1')
+                    msg += 'The option "zlib.output_compression" is enabled in your PHP configuration. '
+                         + 'Minify set this to "0", but it had no effect. This option must be disabled ' 
+                         + 'in php.ini or .htaccess.';
+                else
+                    msg += 'The option "zlib.output_compression" is disabled in your PHP configuration '
+                         + 'so this behavior is likely due to a server option.';
+                $(document.body).prepend(msg + '</p>');
+            } else
+                if (ocStatus == '1')
+                    $(document.body).prepend('<p class=topNote><strong>Note:</strong> The option '
+                        + '"zlib.output_compression" is enabled in your PHP configuration, but has been '
+                        + 'successfully disabled via ini_set(). If you experience mangled output you '
+                        + 'may want to consider disabling this option in your PHP configuration.'
+                    );
+        });
+    });
+});
+</script>
 <script type="text/javascript">
     // workaround required to test when /min isn't child of web root
     var src = location.pathname.replace(/\/[^\/]*$/, '/_index.js').substr(1);
@@ -127,6 +160,7 @@ $serveOpts = array(
         ,filemtime(dirname(__FILE__) . '/../config.php')
     )
     ,'minifyAll' => true
+    ,'encodeOutput' => $encodeOutput
 );
 ob_end_clean();
 

min/builder/ocCheck.php

@@ -0,0 +1,34 @@
+<?php 
+/**
+ * AJAX checks for zlib.output_compression
+ * 
+ * @package Minify
+ */
+
+// allow access only if builder is enabled
+require dirname(__FILE__) . '/../config.php';
+if (! $min_enableBuilder) {
+    header('Location: /');
+    exit();
+}
+
+if (isset($_GET['hello'])) {
+    // echo 'World!'
+    
+    // try to prevent double encoding (may not have an effect)
+    ini_set('zlib.output_compression', '0');
+    
+    require $min_libPath . '/HTTP/Encoder.php';
+    HTTP_Encoder::$encodeToIe6  = true; // just in case
+    $he = new HTTP_Encoder(array(
+        'content' => 'World!'
+        ,'method' => 'deflate'
+    ));
+    $he->encode();
+    $he->sendAll();
+
+} else {
+    // echo status "0" or "1"
+    header('Content-Type: text/plain');
+    echo (int)ini_get('zlib.output_compression');
+}

min/index.php

@@ -9,6 +9,9 @@
 
 define('MINIFY_MIN_DIR', dirname(__FILE__));
 
+// try to disable output_compression (may not have an effect)
+ini_set('zlib.output_compression', '0');
+
 // load config
 require MINIFY_MIN_DIR . '/config.php';