pdfadapterinterface.php

<?php

interface PdfAdapterInterface {
  public function combinePdf($pdf1,$pdf2);
}

pdflibadapter.php

<?php

class PdfLibAdapter implements PdfAdapterInterface {
  //implementation
}

pdfservice.php

<?php

class PdfService implements PdfAdapterInterface {
  
  protected $adapter;
  
  public function __construct(PdfAdapterInterface $adapter){
    $this->adapter = $adapter;
  }
  
  public function combinePdf($pdf1,$pdf2){
    return $this->adapter->combinePdf($pdf1,$pdf2);
  }
  
}