This is a hint rather than an answer. You write
Ctrl+Click on the class name [in the class diagram] in my IDE (PhpStorm), I expect to go to its definition in my code.
I do this with svg-files in a special web-browser of the MATLAB IDE. This statement in the puml-script defines the hyperlink.
class SuperClass <<(H,#E6FFE6)>> [[matlab:matlab.desktop.editor.openAndGoToLine('D:\m\m2uml_sandbox\tests\m4test\+F2\SuperClass.m',1);{No tooltip}]] {
"matlab:matlab.desktop.editor.openAndGoToLine" is MATLAB specific. I'm not familiar with PhpStorm.
See https://plantuml.com/link and https://plugins.jetbrains.com/plugin/14821-plantuml-studio and https://www.jetbrains.com/phpstorm/?q=plantuml&s=full