How to fill an echoed <select> with PHP, after clicking a button - php

Question

I have this problem, I'm using PHP to create an HTML document, creating everything with echo, my problem is that I'm creating a select, but it must be filled when the index of the other one changes, filling in doc creation is easy, but how can I fill it using PHP when selecting something in the other select?
$html_ciudad = '';
$file = $DOCUMENT_ROOT. "newContact.php";
$dom = new DOMDocument;
$dom->loadHTMLFile($file);
$div = $dom->getElementById("City");
foreach($ciudades->getCiudad($id) as $ciu){
$newDiv = $dom->createElement('option');
$newDiv->setIdAttribute($ciu["IdCd"]);
$newDiv->nodeValue(utf8_encode($ciu["Cd"]));
$div->appendChild($newDiv);
$html_ciudad .=
'<option id="'.$ciu["IdCd"].'" value="">'.utf8_encode($ciu["Cd"]).'</option>';
}
I tried that way, everything working, but not showing result, If someone knows how to do this, even if the code is completely different, is welcome haha thanks.

Answer 1

So if i understood it well, if your select value change, you should load from another page with ajax another select...This code may help:
in your main page:
$('select').on('change', function() {
if(this.value==1){
$("#div1").load("another_select.php");
}
})
</script>
in another_select.php page:
echo "<select>";
...
echo "</select>";