PHP If radio button selected, fill text field

I have been trying to figure this little problem out and am about ready to pull my hair out lol. I am more of a designer than I am a programmer, but I have created a submit form in php for my school. Here is the test form I am working on:

http://ift.tt/1zQ7Hgc

What I am trying to do is when a person selects External user, they should be required to fill out their Company Name. When someone selects Internal user, they are required to fill out their Group/PI name. Well this isn't working. When I select Internal, it's supposed to require only Group/PI name and not Company name. I would appreciate any assistance. Thank you!

Here is my PHP code here:

// internal selected (group pi)

 if ($userType == 'internal') {
    $internalChecked = ' checked="checked" ';
    } else {

    if(trim($_POST['grpPI']) === '') {
        $grpPIError = '<span class="error">Please enter your group or PI name.</span>';
        $hasError = true;
    } else {
        $grpPI = trim($_POST['grpPI']);
    }

        }

    }

// external selected (company name)

 if ($userType == 'external') {
    $externalChecked = ' checked="checked" ';
    } else {

    if(trim($_POST['companyName']) === '') {
        $companyNameError = '<span class="error">Please enter your company name.</span>';
        $hasError = true;
    } else {
        $companyName = trim($_POST['companyName']);
    }

        }

and HTML here:

<table class="usertype" id="usertype">   

                         <tr>
                           <td colspan="2">
                             <input type="radio" name="userType" value="external" <?php echo $externalChecked; ?>>External 
                             <select name="externalSelect" size="1">
                               <option selected="selected">Select</option>
                               <option>Academic</option>
                               <option>Non-Profit</option>
                               <option>Commercial</option>
                           </select><?php if($externalError != '') { ?><span class="error"><?=$externalError;?></span><?php } ?></td>
           </tr>
                         <tr>
                           <td colspan="2"><label for="companyName">Company Name:</label> 
                           <input type="text" size="40" name="companyName" id="companyName" value="<?php if(isset($_POST['companyName'])) echo $_POST['companyName'];?>" />
                <?php if($companyNameError != '') { ?>
                <span class="error"><?=$companyNameError;?></span><?php } ?></td>
                         </tr>
                         <tr>
                           <td colspan="3"><input type="radio" name="userType" value="internal" <?php echo $internalChecked; ?>>Internal &nbsp;&nbsp;&nbsp;&nbsp;<input title="Enter Account String. Required for Internal Users." type="text" size="30" name="ucfund" value="<?php if(isset($_POST['ucfund'])) echo $_POST['ucfund'];?>" placeholder="KFS or Account (eg. xxxxxxx)" /><?php if($ucfundError != '') { ?><span class="error"><?=$ucfundError;?></span><?php } ?>&nbsp;-&nbsp; <input title="Enter Fund String. Required for Internal Users." type="text" size="25" name="ucfund2" value="<?php if(isset($_POST['ucfund2'])) echo $_POST['ucfund2'];?>" placeholder="Fund (eg. xxxxx)" /><?php if($ucfund2Error != '') { ?><span class="error"><?=$ucfund2Error;?></span><?php } ?><br /><span class="small">(Please enter KFS number or Accound Fund)</span>
</td>
                         </tr>

                         <tr>
                           <td colspan="3">Optional&nbsp;&nbsp;&nbsp;&nbsp;<input type="text" size="30" name="sub" value="<?php if(isset($_POST['sub'])) echo $_POST['sub'];?>" placeholder="Sub" /><?php if($subError != '') { ?><span class="error"><?=$subError;?></span><?php } ?>&nbsp;-&nbsp; <input type="text" size="25" name="proj" value="<?php if(isset($_POST['proj'])) echo $_POST['proj'];?>" placeholder="Project" /><?php if($projError != '') { ?><span class="error"><?=$projError;?></span><?php } ?>
</td>
                         </tr>
                         <tr>
                           <td colspan="3"><label for="grpPI">Group / PI:</label> 
                           <input type="text" size="40" name="grpPI" id="grpPI" value="<?php if(isset($_POST['grpPI'])) echo $_POST['grpPI'];?>" />
                <?php if($grpPIError != '') { ?>
                <span class="error"><?=$grpPIError;?></span><?php } ?></td>
                </tr>

      </table>