How to get the text of dropdown and echo in php - php

Question

I need to get the text of dropdown and use this to get the first 3 letters of the text. I tried to get the value of row category but it always get the last value in the database.
Help?
<?php
$resultcode = $mysqli->query("SELECT category, id, maincode FROM category GROUP BY id ORDER BY maincode");
$code = '';
while($row = $resultcode->fetch_assoc())
{
$ok = $row['category'];
$code .= '<option value = "'.$row['maincode'].'">'.$row['category'].'</option>';
}
?>
<br/>
Category
<select name="supplier" style="text-transform:uppercase;">
<option value="">Select</option>
<?php echo $code; ?>
</select>
<?php
$myStr = $ok;
// singlebyte strings
$result = substr($myStr, 0, 2);
// multibyte strings
$result1 = mb_substr($myStr, 0, 5);
echo $result.'-'.$result1;
?>

Answer 1

You may try something like this:
Category
<select name="supplier" style="text-transform:uppercase;" onchange = "GetChangedValue(this);">
<option value="">Select</option>
<?php echo $code; ?>
</select>
<script>
function GetChangedValue(e) {
var value = e.options[e.selectedIndex].text;
alert(value);
}
</script>

Answer 2

To get the selected text, you can use Javascript
var element = document.getElementById("mySelectTag");
var selected_text = element.options[element.selectedIndex].text;
To get a substring of a text, use Javascript, for example:
selected_text.substring(0,3)

Answer 3

function change(){
var selectedtext = document.getElementById("idselect").value;
document.getElementById("myTextField").value = selectedtext.substring(0,3);
}
<h1 id="title">Javascript example</h1>
<input type="text" id="myTextField"/>
<select id="idselect" onchange="change()">
<option>Choose one</option>
<option value="BLUE">Blue</option>
<option value="YELLOW">Yellow</option>
<option value="RED">Red</option>
</select>
http://jsfiddle.net/Fx5T8/