Flatten FDF / XFDF forms to PDF in PHP with utf-8 characters

You could try the trial version of http://www.adobe.com/products/livecycle/designer/ and see what PDF files it generates.

Another commercial software you could try is http://www.appligent.com/fdfmerge. See page 16 in http://146.145.110.1/docs/userguide/FDFMergeUserGuide.pdf for how it handles xFDF with UTF-8.

I also had a look at the FDF specification http://partners.adobe.com/public/developer/en/xml/xfdf_2.0.pdf On page 12 it states:

Although XFDF is encoded in UTF-8, double byte characters are encoded as character references when 
exported from Acrobat. 
For example, the Japanese double byte characters ,  , and  are exported to XFDF using 
three character references. Here is an example of double byte characters in a form field: 
  ...
<fields>  
  <field name="Text1"> 
     <value>Here are 3 UTF-8 double byte  
        characters: &#x3042;&#x3044;&#x3046;
</value>  
  </field>  
</fields> ... 

I looked through pdftk-1.44-dist/java/com/lowagie/text/pdf/XfdfReader.java. It doesn't seem to do anything special with the input.

Maybe pdftk will do what you want, when you encode the weird characters as character references in your xFDF input.

I found by using Jon's template but using the DomDocument the numeric encoding was handled for me and worked well. My slight variation is below:

$xml = new DOMDocument( '1.0', 'UTF-8' );

$rootNode = $xml->createElement( 'xfdf' );
$rootNode->setAttribute( 'xmlns', 'http://ns.adobe.com/xfdf/' );
$rootNode->setAttribute( 'xml:space', 'preserve' );
$xml->appendChild( $rootNode );

$fieldsNode = $xml->createElement( 'fields' );
$rootNode->appendChild( $fieldsNode );

foreach ( $fields as $field => $value )
{
    $fieldNode = $xml->createElement( 'field' );
    $fieldNode->setAttribute( 'name', $field );
    $fieldsNode->appendChild( $fieldNode );

    $valueNode = $xml->createElement( 'value' );
    $valueNode->appendChild( $xml->createTextNode( $value ) );
    $fieldNode->appendChild( $valueNode );
}

$xml->save( $file );