apply StyleCI recommendation

tst/ConfigurationTest.php

@@ -120,14 +120,15 @@ class ConfigurationTest extends TestCase
         unset($options['expire_options']['1week']);
         unset($options['expire_options']['1year']);
         unset($options['expire_options']['never']);
-        $valid_sri = $options['sri'][array_key_first($options['sri'])];
-        $options['sri'][array_key_first($options['sri'])] = ''; // empty string should get replaced with default
+        $sri_key                         = array_key_first($options['sri']);
+        $valid_sri                       = $options['sri'][$sri_key];
+        $options['sri'][$sri_key]        = ''; // empty string should get replaced with default
         $options['sri']['js/example.js'] = 'some invalid SRI hash';
         Helper::createIniFile(CONF, $options);
         $conf                         = new Configuration;
         // restore expected results
         $options['expire']['default'] = '5min';
-        $options['sri'][array_key_first($options['sri'])] = $valid_sri;
+        $options['sri'][$sri_key]     = $valid_sri;
         $this->assertEquals($options, $conf->get(), 'not overriding "missing" subkeys');
     }