If you're using dompdf 0.6.1 then you should just be able to use the character as-is so long as you style the text to use one of the DejaVu fonts. E.g.
<div style="font-family: DejaVu Sans, sans-serif;">✔</div>
You can also do that with any font that supports the character.
That character is also included as part of the Zapf Dingbats font included in all PDF readers. To display a tick symbol using that font do something like the following:
<div style="font-family: ZapfDingbats, sans-serif;">4</div>
(Though there may be a problem with Zapf support in 0.6.x).