I posted the complete .php script at the end of the OP in here.
When starting the script from the commandline with “0,8” I get:
Message:
Unable to set data for featureheating.circuits.0.heating.curve and action setCurve and data{"shift":4,"slope":0,8}
Reason: Invalid request payload JSON format
Code: 1
Trace:#0 phar:///etc/openhab2/scripts/Viessmann-Api-1.3.0-SNAPSHOT.phar/src/API/ViessmannAPI.php(127): Viessmann\API\ViessmannAPI->setRawJsonData('heating.circuit...', 'setCurve', '{"shift":4,"slo...')
#1 /etc/openhab2/scripts/SetSlopeAndShift.php(21): Viessmann\API\ViessmannAPI->setCurve('4', '0,8')
#2 {main}
Note: The actual script does take two values (4 and 0.8).
It looks to me as if executeCommandLine “transforms” the second commandline argument from localized 0,8 to 0.8!
No calculation is done (IMHO!) with the value.
The small php script is using another .phar, not created by me. Asking the author is the next step.