if(isset($_POST['submit_form'])){
// check if character exists
$checkChar = mysql_query("SELECT * FROM `players` WHERE name='".$_POST['character_name']."' ");
$r = mysql_fetch_assoc($checkChar);
if(!$r){ echo 'A character with this name does not exist!';
} else {
// add item to players depot
$itemID = 'XXXX';
$itemCount = '1';
$addItem = mysql_query("INSERT INTO `player_depotitems` (player_id, item_type, count) VALUES ('".$r['player_id']."', '".$itemID."', '".$itemCount."')");
echo 'Item has been added to player <b>'.$_POST['character_name'].'</b> depot.';
}
}
<form action="" method="POST">
<input type="text" name="character_name" size="10"/>
<input type="submit" name="submit_form" value="Send item"/>
</form>