How can I display a PHP foreach loop’s answers by AJAX

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Your code is selecting from the database where cor_estado.estado_id equals $Estado_id = $_POST["selectVal"];, but in your JavaScript you’re not sending an ID as $_POST["selectVal"], you’re literally sending the text string "selectVal":

data:{action:'my_action', selectVal: 'selectVal'},

Change that to this, to send the actual variable value:

data:{action:'my_action', selectVal: selectVal},