无法从下拉列表中获取选定值到php变量，并使用该变量的值来获取其他数据

I have a form that has some text fields and 2 drop-down lists, from the first drop-down list I need to fetch the selected value and store it in a PHP variable so I can use that variable to fetch another set of data into the 2nd drop-down. I do not want to use the submit button. I have tried using ajax, I am getting the value as chosen in the console, but not able to fetch the same into a PHP variable.I have a form that has some text fields and 2 d