Code cleanup and ensure checked file is unique.

src/Composer/Console/Application.php

@@ -150,9 +150,8 @@ class Application extends BaseApplication
 
             // Check system temp folder for usability as it can cause weird runtime issues otherwise
             Silencer::call(function() {
-                $tempfile = sys_get_temp_dir() . '/tempchk.tmp';
-                $usable = file_put_contents($tempfile, __FILE__) && (file_get_contents($tempfile) == __FILE__) && unlink($tempfile);
-                if (!$usable || file_exists($tempfile)) {
+                $tempfile = sys_get_temp_dir() . '/temp-' . md5(microtime());
+                if (!(file_put_contents($tempfile, __FILE__) && (file_get_contents($tempfile) == __FILE__) && unlink($tempfile) && !file_exists($tempfile))) {
                     throw new \RuntimeException(sprintf('PHP temp directory "%s" does not exist or is not writable to Composer - check sys_temp_dir in your php.ini', sys_get_temp_dir()));
                 }
             });