Guard against mbstring func_overload, fixes #5218

src/Composer/Downloader/GzipDownloader.php

@@ -76,7 +76,7 @@ class GzipDownloader extends ArchiveDownloader
         $archiveFile = gzopen($file, 'rb');
         $targetFile = fopen($targetFilepath, 'wb');
         while ($string = gzread($archiveFile, 4096)) {
-            fwrite($targetFile, $string, strlen($string));
+            fwrite($targetFile, $string, Platform::strlen($string));
         }
         gzclose($archiveFile);
         fclose($targetFile);

src/Composer/Util/Platform.php

@@ -26,4 +26,22 @@ class Platform
     {
         return defined('PHP_WINDOWS_VERSION_BUILD');
     }
+
+    /**
+     * @param  string $str
+     * @return int return a guaranteed binary length of the string, regardless of silly mbstring configs
+     */
+    public static function strlen($str)
+    {
+        static $useMbString = null;
+        if (null === $useMbString) {
+            $useMbString = function_exists('mb_strlen') && ini_get('mbstring.func_overload');
+        }
+
+        if ($useMbString) {
+            return mb_strlen($str, '8bit');
+        }
+
+        return strlen($str);
+    }
 }

src/Composer/Util/RemoteFilesystem.php

@@ -277,7 +277,7 @@ class RemoteFilesystem
         try {
             $result = file_get_contents($fileUrl, false, $ctx);
 
-            if ($this->bytesMax && strlen($result) < $this->bytesMax) {
+            if ($this->bytesMax && Platform::strlen($result) < $this->bytesMax) {
                 // alas, this is not possible via the stream callback because STREAM_NOTIFY_COMPLETED is documented, but not implemented anywhere in PHP
                 throw new TransportException('Content-Length mismatch');
             }