I need to take certain variables from an output and do something with them to build my own Nagios-plugin.

I'm not good with gawk and sed to filter the output, so writing my Nagios-plugin in bash is not an option.

What I do know is php, but for building websites.

Can I use php also inside a script ???

It would then start with #!/bin/php ??

How can I execute a linux-command inside this php-script ??

I want to execute /usr/sbin/asterisk -rx "sip show peers",
which gives me a line for every sip-peer.

I need to filter a specified sip-peer (will be {ARG1}) from this output.

So, do I learn gawk & sed, or can I stick to my php ??

Yes you can use php for your script. If you write a script in php, use the '<?' and '?>' start and end tag. Then, if your file is named sample.php, run it with 'php -f sample.php'.
If you want to execute a command in php, use the function 'exec' of 'passthru' depending on the result you want to get.

Anyway, using php is up to your choice. It certainly supports text filtering, but with sed you can write such filters much faster, so maybe learning it could be a good thing.

And how can I use a PHP-argument inside the exec-statement ??

$PEER=$argv[1];
echo $PEER;
$LINE=exec('/usr/sbin/asterisk -rx "sip show peers" | grep -w "$PEER"');
echo "$LINE";

$PEER is not substituted by its value.

Are you sure? In my system, $PEER has been substituted.
Maybe something else is wrong?