Ability to add processing instructions (#168)

README.md

@@ -477,6 +477,27 @@ This will result in:
 <soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope/"><soap:Header/><soap:Body><soap:key>soap:value</soap:key></soap:Body></soap:Envelope>
 ```
 
+### Adding Processing Instructions
+
+Call `$arrayToXml->addProcessingInstruction($target, $data)` method on ArrayToXml object to prepend a processing instruction before the root element.
+
+Example:
+
+```php
+
+$arrayToXml = new ArrayToXml($array);
+$arrayToXml->addProcessingInstruction('xml-stylesheet', 'type="text/xsl" href="base.xsl"');
+$result = $arrayToXml->toXml();
+```
+
+This will result in:
+
+```xml
+<?xml version="1.0"?>
+<?xml-stylesheet type="text/xsl" href="base.xsl"?>
+<root><Good_guy><name>Luke Skywalker</name><weapon>Lightsaber</weapon></Good_guy><Bad_guy><name>Sauron</name><weapon>Evil Eye</weapon></Bad_guy></root>
+```
+
 ## Testing
 
 ```bash

src/ArrayToXml.php

@@ -123,6 +123,17 @@ public function dropXmlDeclaration(): self
         return $this;
     }
 
+    public function addProcessingInstruction($target, $data)
+    {
+        $elements = $this->document->getElementsByTagName('*');
+
+        $rootElement = $elements->count() > 0 ? $elements->item(0) : null;
+
+        $this->document->insertBefore($this->document->createProcessingInstruction($target, $data), $rootElement);
+
+        return $this;
+    }
+
     protected function convertElement(DOMElement $element, mixed $value): void
     {
         $sequential = $this->isArrayAllKeySequential($value);

tests/ArrayToXmlTest.php

@@ -515,7 +515,7 @@ public function it_can_drop_xml_declaration()
 
         $this->assertMatchesSnapshot($arrayToXml->dropXmlDeclaration()->toXml());
     }
-
+    
     /** @test */
     public function it_can_convert_an_array_with_null_value_to_xml()
     {
@@ -525,4 +525,14 @@ public function it_can_convert_an_array_with_null_value_to_xml()
 
         $this->assertMatchesXmlSnapshot(ArrayToXml::convert($arr));
     }
+
+    /** @test */
+    public function it_can_add_processing_instructions()
+    {
+        $arrayToXml = new ArrayToXml($this->testArray);
+
+        $arrayToXml->addProcessingInstruction('xml-stylesheet', 'type="text/xsl" href="base.xsl"');
+
+        $this->assertMatchesSnapshot($arrayToXml->toXml());
+    }
 }

tests/__snapshots__/ArrayToXmlTest__it_can_add_processing_instructions__1.txt

@@ -0,0 +1,3 @@
+<?xml version="1.0"?>
+<?xml-stylesheet type="text/xsl" href="base.xsl"?>
+<root><Good_guy><name>Luke Skywalker</name><weapon>Lightsaber</weapon></Good_guy><Bad_guy><name>Sauron</name><weapon>Evil Eye</weapon></Bad_guy></root>