Another approach that worked for me (Ubuntu 16.04 / Ispconfig 3.1.2). Keep SPAM check and disablie clamav to free up memory....
thanks vistree...really helped.:)
Have you tried to enable all deb-src urls in your sources.list file and then run apt-get build-dep php7.0 ?
Had the same problem too. Tried to install additional PHP 5.6.11 on a Ubuntu 16.04 / PHP 7.0 and use it in my ISPConfig for some older sites....
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