How to enable a field if a determined option is chosen? - php

Question

I have a form in which when the user chooses certain option in a select drop-down, a new field should shows up for entering another piece of information.
Basically, it's the common "Where did you hear about us?" and when the user selects other, a new empty field is shown so that he can type the source if it wasn't in the list.
But I really have no clue about how to do it. I've been using PHP and HTML to do the form and validating it.
To add some code, this is where I create the select
<label for="where_heard">Where heard:</label>
<select name="where_heard"> <?php echo $sources; ?>
</select>
And this is where I get the source list from:
$query = "SELECT source FROM where_heard";
$stmt = $db->prepare($query);
$stmt->execute();
$sources = "";
while ($row = $stmt->fetch()) {
$sources .= '<option value= "'.$row['source'].'">'.$row['source'].'</option>';
}

Answer 1

You can do this with jquery like:
$( document ).ready(function() {
$('select').on("change",function(){
if($(this).val() == "other")
$(".other").html("Other: <input type='text' name='other'/>");
else
$(".other").html("");
})
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<label for="where_heard">Where heard:</label>
<select name="where_heard"><option></option><option value="other">Other</option>
</select>
<div class="other"></div>