[BUGFIX] Silence purposefully ignored PHP warnings (#400)

CHANGELOG.md

@@ -28,6 +28,8 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 
 ### Fixed
+- Silence purposefully ignored PHP Warnings
+  ([#400](https://github.com/MyIntervals/emogrifier/pull/400))
 
 
 ### Security

Classes/Emogrifier.php

@@ -373,10 +373,17 @@ protected function process(\DOMDocument $xmlDocument)
         $excludedNodes = $this->getNodesToExclude($xPath);
         $cssRules = $this->parseCssRules($cssParts['css']);
         foreach ($cssRules as $cssRule) {
-            // query the body for the xpath selector
-            $nodesMatchingCssSelectors = $xPath->query($this->translateCssToXpath($cssRule['selector']));
-            // ignore invalid selectors
-            if ($nodesMatchingCssSelectors === false) {
+            // There's no real way to test "PHP Warning" output generated by the following XPath query unless PHPUnit
+            // converts it to an exception. Unfortunately, this would only apply to tests and not work for production
+            // executions, which can still flood logs/output unnecessarily. Instead, Emogrifier's error handler should
+            // always throw an exception and it must be caught here and only rethrown if in debug mode.
+            try {
+                // \DOMXPath::query will always return a DOMNodeList or an exception when errors are caught.
+                $nodesMatchingCssSelectors = $xPath->query($this->translateCssToXpath($cssRule['selector']));
+            } catch (\InvalidArgumentException $e) {
+                if ($this->debug) {
+                    throw ($e);
+                }
                 continue;
             }
 
@@ -1535,7 +1542,9 @@ private function getNodesToExclude(\DOMXPath $xPath)
      */
     public function handleXpathError($type, $message, $file, $line, array $context)
     {
-        if ($this->debug && $type === E_WARNING && isset($context['cssRule']['selector'])) {
+        // Do not tie this conditional to debug mode as it causes unnecessary output. See further explanation in
+        // ::process() where this exception is caught.
+        if ($type === E_WARNING && isset($context['cssRule']['selector'])) {
             throw new \InvalidArgumentException(
                 sprintf(
                     '%s in selector >> %s << in %s on line %s',