<?php
    header('Content-Type: application/json');
    echo json_encode(array('One' => 'Test'));
?>

<script type = "text/javascript">
    let changeValue;
    $('#someElementID').click(function() {
        $.ajax({
            type: 'POST',
            url : 'phpfile.php',
            dataType : 'json',
            success: function(obj, textStatus) {
                changeValue = obj.One;
            }
        });

        $('#toBeChangedElementValue').text(changeValue);
    }
</script>