How to Capture Szimek/Signature_Pad with PHP (Capture Javascript into PHP Variable)?

How to Capture Szimek/Signature_Pad with PHP (Capture Javascript into PHP
Variable)?

I was browsing StackOverflow and came across Szimek/Signature_Pad to
capture electronic / digital signatures using Javascript.
I researched and I'm still dumbfounded on how to capture the DATA URI into
a variable.
http://szimek.github.io/signature_pad/
I want to capture it like this $inputESignature =
signaturePad.toDataURL(), where signaturePad.toDataURL() is Javascript.
If clarification is needed, please let me know. Sorry if my request is a
bit vague.