I am using step debugging via Netbeans with no problems at all , but following the documentation for doing so on the command line I found out that the recommended setting for
set XDEBUG_SESSION=1

will not work. Doing so, the log file for Xdebug shows me this line :
[276] [Step Debug] -> <init xmlns="urn:debugger_protocol_v1" xmlns:xdebug="https://xdebug.org/dbgp/xdebug" fileuri="file:///F:/xampp-htdocs/phpWebSocketServer/ignore/getOptions.php" language="PHP" xdebug:language_version="8.0.4RC1" protocol_version="1.0" appid="276" idekey="1"><engine version="3.0.4"><![CDATA[Xdebug]]></engine><author><![CDATA[Derick Rethans]]></author><url><![CDATA[https://xdebug.org]]></url><copyright><![CDATA[Copyright (c) 2002-2021 by Derick Rethans]]></copyright></init>

Here the idekey equals the value of XDEBUG_SESSION, therefore there is no reaction in Netbeans where I set my breakpoints. The PHP script just hangs.
Setting XDEBUG_SESSION=netbeans-xdebug will make things work, when Xdebug is already started via Netbeans. I could not manage to get Xdebug started from the command line. The record from the log file looks now like:

[11056] [Step Debug] -> <init xmlns="urn:debugger_protocol_v1" xmlns:xdebug="https://xdebug.org/dbgp/xdebug" fileuri="file:///F:/xampp-htdocs/phpWebSocketServer/ignore/getOptions.php" language="PHP" xdebug:language_version="8.0.4RC1" protocol_version="1.0" appid="11056" idekey="netbeans-xdebug"><engine version="3.0.4"><![CDATA[Xdebug]]></engine><author><![CDATA[Derick Rethans]]></author><url><![CDATA[https://xdebug.org]]></url><copyright><![CDATA[Copyright (c) 2002-2021 by Derick Rethans]]></copyright></init>

Am I missing something ?