How to config new Apache path for PHP on Ubuntu 16.04?

Question

I want to install Apache version 2.4.28 and PHP version 5.6 on Ubuntu 16.04.

I installed successfully but they did not work together because when I install apache 2.4.28 I can't use:

apt-get install apache2----> this commmand just install apache 2.4.18.

Instead, I compile apache 2.4.28 from source as link.

Then I use apt-get to install PHP 5.6. Because default PHP for Ubuntu 16.04 is PHP7.0, I use this command to install PHP5.6:

apt-get install software-properties-common
add-apt-repository ppa:ondrej/php
apt-get update
apt-get install php5.6

And it installed success!

So, when I test PHP:

<?php
    echo "Hello world";
?>

It show blank page, not print "Hello world".

root@ubuntu:/usr/local/apache2/conf# ps -aux | grep httpd
root      57261  0.0  0.1  75276  3844 ?        Ss   08:21   0:00 /usr/local/apache2/bin/httpd -k start
daemon    57262  0.0  0.2 364240  5760 ?        Sl   08:21   0:00 /usr/local/apache2/bin/httpd -k start
daemon    57263  0.0  0.2 364240  5760 ?        Sl   08:21   0:00 /usr/local/apache2/bin/httpd -k start
daemon    57264  0.0  0.3 429776  6100 ?        Sl   08:21   0:00 /usr/local/apache2/bin/httpd -k start
root      57647  0.0  0.0  21296   944 pts/17   S+   08:55   0:00 grep --color=auto httpd

I think it many something wrong in configure PHP path or apache path. But I don't know how to fix it. Please help me. Thanks in advance.