Send big amouts of data to PHP script via gsm module SIM800L
Do you have any idea how I can send bigger amounts of data all at once? For energy saving purposes I would prefer if it would be possible in only a single request.


Do a POST request. You can send huge amounts of data in the body.


  In the response from my SIM800l I get back a string with exactly 63 characters, so I assume there's some lenght limitation there on the SIM-module.


Nope.

I guess you're doing something like:

sendCommand("AT+HTTPPARA=URL,www.example.com/newData.php?data=t0_20.75,l_0.00,m_584,h_42,td_22,d_21x44x54");
delay(1000);
printResponse();


The problem there is that you're relying on the internal buffer of SoftwareSerial (or HardwareSerial if you use a real serial port). You wait for a predefined time for characters to arrive, and then after that time print them out.  The serial code has a 64 byte buffer. 

I see that kind of code all over the place, and it's the worst possible way of reading serial. Whoever posted the first instance of such code and started this chain of evil should be taken outside and shot.

Instead you must read the serial properly. Being lazy like that doesn't work.

Read this blog post to learn more about reading serial data properly.