Using Onchange behaviour of a dropdown list to submit a form

I want to amend my page so that depending on the value selected in a drop down list the page will display fields relevant to the selected value only. I am assuming the best way to do this is to submit the form onchange of the dropdown, and display the fields I want depending on what value was selected. My question is how to I get the form to submit ? I am using PHP/Javascript and a MySQL database.
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This is pretty straight forward
The select tag looks something like this
<SELECT id='sel1' onchange='this.form.submit();' >
the value selected will be in the $_POST['sel1'] variable

you could make your javascript a bit cooler by hidding and showing the relevant fields without the page select

A simple but clunky way to do this would be

function show_fields(sel1_val){
if(sel1_val == '2and3'){
//blank the hidden fields so they wont get submitted
document.getElementById('field_1').value ='';
//show the right fields
document.getElementById('field_1').style.visiblity ='hidden';
document.getElementById('field_2').style.visiblity ='visible';
document.getElementById('field_3').style.visiblity ='visible';
document.getElementById('field_1').style.visiblity ='visible';
//blank the hidden fields so they wont get submitted
document.getElementById('field_2').value ='';
document.getElementById('field_3').value ='';
//show the right fields
document.getElementById('field_2').style.visiblity ='hidden';
document.getElementById('field_3').style.visiblity ='hidden';



the the select tag looks like this

<SELECT id='sel1' onchange='show_fields(this.options[this.selectedIndex].value;);' >
There are lots of way to do this but thats a couple of suggestions
<form name='frm' >
<select onchange="document.getElementByID('frm').submit();">

i think for showing fields onchange event, submitting the form is not a good idea.
us a javascript way. means hidding and showing divs as suggested by spotx

 or even better way is to use ajax to do this.
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