[PHP7.2] "A non well formed numeric value encountered" in length conversion

Hey,

we're using the purifier to clean up HTML mails before display. We recently upgraded to PHP 7.2 and now got a mail (from Outlook 😩 ) which runs into a notice which breaks our app.

The error is thrown in vendor/ezyang/htmlpurifier/library/HTMLPurifier/Length.php while comparing 2 lengths:

HTMLPurifier_Length {#7557 ▼
  #n: ".5208"
  #unit: "in"
  #isValid: true
}

and

HTMLPurifier_Length {#7645 ▼
  #n: "1200"
  #unit: "px"
  #isValid: null
}

At the end he tries to substract ".5208" - "0,0000" (the latter with a comma) and that throws the notice. I've dug into the to UnitConverter, but the comma is all the time around. We don't have bcmath installed. After installing that he correctly converts the 1200px into "12.5" and doesn't explode.

Cheers
Matthias

Start in vendor/ezyang/htmlpurifier/library/HTMLPurifier/Length.php, where the comparison triggers the notice, and trace the referenced UnitConverter conversion for .5208in and 1200px without bcmath. Reproduce the PHP 7.2 case, then verify that length comparison no longer emits the malformed numeric value notice and that the conversion produces a usable numeric result.