Recently I had a crash in PHP and wanted to provide a proper backtrace with my bug report. Simply running gdb did not give me a usable trace, because the debug symbols were missing.
PHP was installed via Ondrej Sury's PHP personal package archive, and I did not see any debug packages there.
The Debugging a Program Crash page in the Ubuntu wiki told me what to do: Add main/debug at the end of the deb line in the PPA's apt sources.list.d file:
deb http://ppa.launchpad.net/ondrej/php/ubuntu trusty main main/debug
After apt-updating, I could install php7.1-common-dbgsym and php7.1-cli-dbgsym and get a nice backtrace.