SigToSvg is a project mainly written in PHP, it's free.
Convert jQuery Signature Pad plugin data to an SVG Image
Signature to SVG Image: a supplemental server-side script for "Signature Pad" that generates a scalable vector graphic (SVG) of the Signature Pad's output. PHP 5.3 and above required but no other external dependencies (like GD).
Thanks to Thomas Bradley: http://thomasjbradley.ca/lab/signature-pad