PHP if option is selected, selected option can't selected again in other chooses

Question

Hello my name is Patrick and this is my first question, i'm sorry but i'm not very good in PHP. probably there are more improvements but this post is for the questions. (but improvements are also welcome)

Question:

You can choose a team of 2 monsters // The monster are selected form database

The question is: if you choose 1 monster how can i fix that you can't choose the same monster on option 2?

PHP CODE:

Action of the 2 sumbit buttons

<?php
session_start();
include("header.php");

if(!isset($_SESSION['uid'])){
    echo "You must be logged in to view this page!";
}else{
    if (isset($_POST['save'])) {
    if ($_POST['save'] == 'keuze4') {

        $fuelQuery4 = sprintf("UPDATE user_team SET `m_keuze4` = '%s' WHERE `id`='".$_SESSION['uid']."'  ",
                        mysql_real_escape_string($_POST['option4']));
        $Result = mysql_query($fuelQuery4);
            if($Result){
            echo 'Team is aangepast!';
            }


    } elseif ($_POST['save'] == 'keuze5'){

        $fuelQuery5 = sprintf("UPDATE user_team SET `m_keuze5` = '%s' WHERE `id`='".$_SESSION['uid']."'  ",
                        mysql_real_escape_string($_POST['option5']));   
        $Result = mysql_query($fuelQuery5);
            if($Result){
            echo 'Team is aangepast!';
            } 
    }

    echo '';}   

?>

Get the monsters form database and put it in a select list

<?php
 $get=mysql_query("SELECT * FROM user_monsters WHERE `id`='".$_SESSION['uid']."' ORDER BY usid ASC");
$option4 = '';
 while($row = mysql_fetch_assoc($get))
{
  $option4 .= '<option value = "'.$row['usid'].'">'.$row['usid'].' - '.$row['monster'].' - '.$row['type'].'</option>';
}
?>

Show the selected item

<?php

 $k4 = mysql_query("


SELECT user_team.m_keuze4, user_monsters.usid, user_monsters.monster, user_monsters.type, user_monsters.attack, user_monsters.defense
FROM user_team
INNER JOIN user_monsters
ON user_team.m_keuze4=user_monsters.usid
ORDER BY user_monsters.type;

 ");
 while($row4 = mysql_fetch_assoc($k4))
 {
 $k4_1 = ''.$row4['m_keuze4'].' - '.$row4['monster'].' - '.$row4['type'].' - '.$row4['attack'].' - '.$row4['defense'].'';
 }

?>

Option 5 is the same code as 4:

    <?php
 $get=mysql_query("SELECT * FROM user_monsters WHERE `id`='".$_SESSION['uid']."' ORDER BY usid ASC");
$option5 = '';
 while($row = mysql_fetch_assoc($get))
{
  $option5 .= '<option value = "'.$row['usid'].'">'.$row['usid'].' - '.$row['monster'].' - '.$row['type'].'</option>';
}
?>

<?php

 $k5 = mysql_query("


SELECT user_team.m_keuze5, user_monsters.usid, user_monsters.monster, user_monsters.type, user_monsters.attack, user_monsters.defense
FROM user_team
INNER JOIN user_monsters
ON user_team.m_keuze5=user_monsters.usid
ORDER BY user_monsters.type;

 ");
 while($row5 = mysql_fetch_assoc($k5))
 {
 $k5_1 = ''.$row5['m_keuze5'].' - '.$row5['monster'].' - '.$row5['type'].' - '.$row5['attack'].' - '.$row5['defense'].'';
 }

?>

The Form

<form action="team.php" method="post">

 <select name="option4"> 
<?php echo $option4; ?>
</select><br><br>Keuze 4  
<?php 
 echo $k4_1;
  ?><br><br>
<input type="submit" name="save" value="keuze4"/>
</form>

<form action="team.php" method="post">
 <select name="option5"> 
<?php echo $option5; ?>
</select><br><br>Keuze 5  
<?php 
 echo $k5_1;
  ?><br><br>
<input type="submit" name="save" value="keuze5"/>
</form>

Answer 1

In php the best you can do check the option once its posted:

if (isset($_POST['save'])) {
  if (filter_input(INPUT_POST,'option4') == filter_input(INPUT_POST,'option5')){
    echo "Sorry. You can't select the same monster twice";
  }else{
    //your db insert logic goes here
  }
}

It would be a good idea to also include some javascript to alert the user before they submit the form. This example uses jQuery

$('[name="option4"],[name="option5"]').change(function(){
   if  ($('[name="option4"]').val() == $('[name="option5"]').val()){
      alert('you already chose that monster, please choose another');
   }
});

The Form

<form action="team.php" method="post">

    <select name="option4"> 
        <?php echo $option4; ?>
    </select><br><br>Keuze 4  
    <?php
    echo $k4_1;
    ?><br><br>
    <input type="submit" name="save" value="keuze4"/>
</form>                                 <!-- remove this line-->

<form action="team.php" method="post">  <!-- and    this line-->
    <select name="option5"> 
        <?php echo $option5; ?>
    </select><br><br>Keuze 5  
    <?php
    echo $k5_1;
    ?><br><br>
    <input type="submit" name="save" value="keuze5"/>
</form>

<script src="//ajax.googleapis.com/ajax/libs/jquery/1.11.2/jquery.min.js"></script>

<script>
    $(function () {
        $('[name="option4"],[name="option5"]').change(function () {
            if ($('[name="option4"]').val() == $('[name="option5"]').val()) {
                alert('you already chose that monster, please choose another');
            }
        });
    });
</script>

Action of the 2 sumbit buttons

 if (isset($_POST['save'])) {
        if (filter_input(INPUT_POST, 'option4') == filter_input(INPUT_POST, 'option5')) {
            echo "Sorry. You can't select the same monster twice";
        } else {
            if ($_POST['save'] == 'keuze4') {

                $fuelQuery4 = sprintf("UPDATE user_team SET `m_keuze4` = '%s' WHERE `id`='" . $_SESSION['uid'] . "'  ", mysql_real_escape_string($_POST['option4']));
                $Result = mysql_query($fuelQuery4);
                if ($Result) {
                    echo 'Team is aangepast!';
                }
            } elseif ($_POST['save'] == 'keuze5') {

                $fuelQuery5 = sprintf("UPDATE user_team SET `m_keuze5` = '%s' WHERE `id`='" . $_SESSION['uid'] . "'  ", mysql_real_escape_string($_POST['option5']));
                $Result = mysql_query($fuelQuery5);
                if ($Result) {
                    echo 'Team is aangepast!';
                }
            }
        }
    }

Edit again,

Demo Fiddle of js