"squirrelmail" package cannot be installed due to missing dependencies

[Lars Kellogg-Stedman]

Trying to install "squirrelmail" fails due to a dependency problem (missing php5):

/# apt install squirrelmail
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 squirrelmail : Depends: libapache2-mod-php5 but it is not installable or
                         php5 but it is not installable or
                         php5-cgi but it is not installable
E: Unable to correct problems, you have held broken packages.

[Lars Kellogg-Stedman (larsks)]

(This is Raspbian Stretch, last 'apt update' @ Fri Jan 26 04:37:10 UTC 2018)