Localhost is always responding with “Connection refused: localhost:80”

I've been working with the integrated webserver of OS X El Capitan for a while now.
I am developing a website project and thus, this was the perfect solution for me.
I was starting the local server using php -S localhost:8088 -t . in the specific directory containing the website data (index.php etc.).
Everything was working fine until I refreshed the page one day and it suddenly printed: Connection refused: localhost:8088. There were no changes made to any config files or anything else.
It doesn't matter which port I am using, it always refuses the connection. Sending a request to 127.0.0.1 in my browser tells me It works!. But, once I am starting the server and calling 127.0.0.1:8088, my browser responds with its default page displaying that the connection has been refused, as if I did not start/run the server.

The first thing I did was checking whether the Apache config is broken.
But, apachectl configtest printed Syntax OK.

When I tried to ping localhost, I encountered a 100% package loss, so at first I thought that the bug may be located inside the hosts file.
cat /etc/hosts/ printed out the following:

##
##
# Host Database
#
# localhost is used to configure the loopback interface
# when the system is booting. Do not change this entry.
##
127.0.0.1 localhost
255.255.255.255 broadcasthost
::1 localhost
fe80::1%lo0 localhost

Looks appropriate, doesn't it?
So, I checked my firewall options and discovered that I had "Stealth mode" activated. Unchecking it resulted in having the ICMP-packages finding their way correctly and there was no package loss any longer.

The thing is that I don't know why this problem suddenly occured and how I am able to fix it. I already restored all Apache config files to the originals, but that didn't solve it neither.
It seems to be listening on that port, but I can't access the server as it always refuses the connection.
Even if the server is not started, accessing localhost prints Connection refused: localhost:80. I don't know where the error could be located.
Apache's error log didn't show anything conspicious neither.

sudo lsof -i -n -P | grep TCP | grep LISTEN showed that httpd is using IPv6 only, so that should work.

If you need any more (config) files, just request them.
Thanks!