I recently learned that webmail clients like Gmail will do alterations on HTML emails, for example adding target="_blank" to <a> tags.
I've also discovered that other alterations happen as well. When I send an HTML email to Gmail (and possibly other web mail clients) from my PHP script, variable values included in the URL of any links are being stripped out. So, for example, this is the value I'm setting in my PHP code:
$mailContent = '<p><a target="_blank" href="https://example.com/confirmation.html?verification=x1x1x1x1x1x1x1x&email=yyyy@email.com">click here to go to the web site and activate your account!</a></p>';
But when the email is received in Gmail, the HTML code comes out like this:
<p><a target="_blank" href="https://example.com/confirmation.html?verification=&email=">click here to go to the web site and activate your account!</a></p>
The values x1x1x1x1x1x1x1x and yyyy@email.com have been stripped out from within the <a> tag.
How do I protect the values of the variables that I want to pass to the URL so that Gmail won't remove them?
Click View original/source on the message in Gmail to see if the URLs looks like they should then. If so you know that the problem is how Gmail is formatting the message for your viewing. If it's mutilated even in the source I was wondering if there's anything in your webpage/php/CMS (do you use one) that changes the code.
You should try URL-encoding as @Crisp said. Here's the W3 reference.