Added code to expose part content ID

Author: SteJaySulli

README.md

@@ -47,3 +47,18 @@ foreach($parts['IMAGE/PNG'] as $image) {
     file_put_contents($image->name, $image->content);
 }
 ```
+
+## Content IDs
+
+Some emails embed images into the html content using "cid:" urls; these URLs link to the content ID of another part within the email rather than being a URL that can be resolved normally.
+
+If one exists, this content ID will be exposed via the `contentId` property of the `Part`:
+
+```php
+foreach($parts['IMAGE/PNG'] as $image) {
+    // Store the file as in the above example:
+    file_put_contents($image->name, $image->content);
+    // You would then want to store this relationship in your database:    
+    echo "{$image->contentId} => {$image->name}\n";
+}
+```

src/EmailStructureParser.php

@@ -83,17 +83,19 @@ private function getPart(&$returnParts, $structure, $partNumber = false)
                 {
                     $mimeType = $this->getMimeType($substructure);
                     $name = $this->getParameterValue($substructure, 'name');
+                    $contentId = $this->getPartContentId($substructure);
                     $content = $this->getPartByPartNumber($substructure, $partToGet);
-                    $returnParts[$mimeType][] = new Part($name, $content);
+                    $returnParts[$mimeType][] = new Part($name, $content, $contentId);
                 }
             }
         }
         else
         {
             $mimeType = $this->getMimeType($structure);
             $name = $this->getParameterValue($structure, 'name');
+            $contentId = $this->getPartContentId($structure);
             $content = $this->getPartByPartNumber($structure, 1);
-            $returnParts[$mimeType][] = new Part($name, $content);
+            $returnParts[$mimeType][] = new Part($name, $content, $contentId);
         }
 
     }
@@ -137,6 +139,18 @@ private function getPartByPartNumber($structure, $partToGet)
         }
     }
 
+    /**
+     * @param $structure
+     * @return string
+     */
+    private function getPartContentId($structure) {
+        if(isset($structure->id)) {
+            // Content IDs are wrapped in <>, so we remove these:
+            return preg_replace('/[<>]/', '', $structure->id);
+        }
+        return null;
+    }
+
     /**
      * @param $structure
      * @return string

src/Part.php

@@ -7,9 +7,10 @@ class Part
     public $name;
     public $content;
 
-    public function __construct($name, $content)
+    public function __construct($name, $content, $contentId = null)
     {
         $this->name = $name;
         $this->content = $content;
+        $this->contentId = $contentId;
     }
 }