I ran into the same issue.
A couple of things seem to have been deprecated in php 7 - assigning the return value of new by reference and the mysql plugin.
Here's a couple of patches that I used to get things running under php 7.
The assignment patch removes the deprecated assignment.
The mysqli patch updates the code to use the mysqli plugin.
If anyone is on arch, PKGBUILD.txt is an updated build file.
Running makepkg should build the mythplugins package and create a new mythplugins-mythweb file.
Run something like the following to update the mythweb package.
pacman -U mythplugins-mythweb-1:0.27.5-2-x86_64.pkg.tar.xz
And check that you have the mysqli extension enabled in php.
(1/1) Installiere mythplugins-mythweb [###################################################################] 100%
/tmp/alpm_tpotAV/.INSTALL: Linee 1: !DOCTYPE: File or directory not found
/tmp/alpm_tpotAV/.INSTALL: Line 2: Syntaxerror beim unerwarteten Wort `newline'
/tmp/alpm_tpotAV/.INSTALL: Line 2: ` "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">'
/usr/bin/bash: post_upgrade: command not found.