Added project reference (BT-11)

- Updated Invoice class
- Updated UBL reader and writer
- Updated tests

> Closes #60

src/Invoice.php

@@ -255,6 +255,7 @@ class Invoice {
     protected $salesOrderReference = null;
     protected $tenderOrLotReference = null;
     protected $contractReference = null;
+    protected $projectReference = null;
     protected $paidAmount = 0;
     protected $roundingAmount = 0;
     protected $customVatAmount = null;
@@ -679,6 +680,26 @@ public function setContractReference(?string $contractReference): self {
     }
 
 
+    /**
+     * Get project reference
+     * @return string|null Project reference
+     */
+    public function getProjectReference(): ?string {
+        return $this->projectReference;
+    }
+
+
+    /**
+     * Set project reference
+     * @param  string|null $contractReference Project reference
+     * @return self                           Invoice instance
+     */
+    public function setProjectReference(?string $projectReference): self {
+        $this->projectReference = $projectReference;
+        return $this;
+    }
+
+
     /**
      * Get invoice prepaid amount
      * @return float Invoice prepaid amount

src/Readers/UblReader.php

@@ -183,6 +183,12 @@ public function import(string $document): Invoice {
             $invoice->addAttachment($this->parseAttachmentNode($node));
         }
 
+        // BT-11: Project reference
+        $projectReferenceNode = $xml->get("{{$cac}}ProjectReference/{{$cbc}}ID");
+        if ($projectReferenceNode !== null) {
+            $invoice->setProjectReference($projectReferenceNode->asText());
+        }
+
         // Seller node
         $sellerNode = $xml->get("{{$cac}}AccountingSupplierParty/{{$cac}}Party");
         if ($sellerNode !== null) {

src/Writers/UblWriter.php

@@ -141,6 +141,12 @@ public function export(Invoice $invoice): string {
             $this->addTenderOrLotReferenceNode($xml, $invoice);
         }
 
+        // BT-11: Project reference
+        $projectReference = $invoice->getProjectReference();
+        if ($projectReference !== null) {
+            $xml->add('cac:ProjectReference')->add('cbc:ID', $projectReference);
+        }
+
         // Seller node
         $seller = $invoice->getSeller();
         if ($seller !== null) {

tests/Integration/peppol-base.xml

@@ -51,6 +51,9 @@
             <cbc:EmbeddedDocumentBinaryObject mimeCode="application/pdf" filename="ATT-1234.pdf">VGhlIGF0dGFjaG1lbnQgcmF3IGNvbnRlbnRz</cbc:EmbeddedDocumentBinaryObject>  
         </cac:Attachment>
     </cac:AdditionalDocumentReference>
+    <cac:ProjectReference>
+        <cbc:ID>PROJ-1234</cbc:ID>
+    </cac:ProjectReference>
     <cac:AccountingSupplierParty>
         <cac:Party>
             <cbc:EndpointID schemeID="0088">9482348239847239874</cbc:EndpointID>