send a php variable as part of an html form - php

Question

I am working on a website which I didn't create and to which I do not have full access (I can create a page and add html scripts but not much more)
I am trying to add a form which will send data to be processed by a third-party servlet.
I am trying to include in this form the client ID, which I saw can be retrieved using the php functionsession_id().
My question is, how to send this information along with the rest of the input data.
my form for now is this:
<form class="form-horizontal" action = "url" method="POST" target="_blank">
<fieldset>
<!-- Select Basic -->
<div class="form-group">
<label class="col-md-4 control-label" for="selectbasic">Culture</label>
<div class="col-md-4">
<select id="Crop" name="Crop" class="form-control">
<option value="1">Maïs</option>
<option value="2">Ble</option>
</select>
</div>
</div>
<!-- Select Basic -->
<div class="form-group">
<label class="col-md-4 control-label" for="selectbasic">Produit</label>
<div class="col-md-4">
<select id="Product" name="Product" class="form-control">
<option value="Iodure d'azote">Iodure D'azote</option>
<option value="Desherbant">Desherbant</option>
</select>
</div>
</div>
<!-- Text input-->
<div class="form-group">
<label class="col-md-4 control-label" for="area">Surface</label>
<div class="col-md-4">
<input id="Area" name="Area" type="text" placeholder=" " class="form-control input-md" required="">
<span class="help-block">en Ha</span>
</div>
</div>
<!-- Text input-->
<div class="form-group">
<label class="col-md-4 control-label" for="dose">Dosage</label>
<div class="col-md-4">
<input id="Dose" name="Dose" type="text" placeholder="" class="form-control input-md" required="">
<span class="help-block">en L/Ha</span>
</div>
</div>
<div class="form-group">
<!--need to get the actual id-->
<input type="hidden" value="1" name = "ID"/>
<input type="submit" value="Ajouter" />
</div>
</fieldset>
</form>
Since this is my first time handling web apps, is there something obvious i'm missing?

Answer 1

OK i was lacking basic understanding of php. sorry for bothering,
the answer was here
i just had to write
<input type="hidden" value="<?php echo session_id();?>" name = "ID"/>
instead of
<input type="hidden" value="1"name = "ID"/>