Hente data fra form via for-løkke

Jeg roder med et script der skal hjælpe brugeren med at oprette en tabel. Brugeren angiver hvor mange felter der skal være i tabellen, hvorefter følgende script genererer input formen:

******FORM SCRIPT START*********
echo "<form action='dbadmin.php?action=create_table_do' method=post onsubmit='return ValiderForm_Create_Step2(this);'>";

echo "<input type=hidden name=table_name value='".$_POST['table_name']."'>";
echo "<input type=hidden name=number_fields value='".$_POST['number_fields']."'>";

echo "<table align=center>";
for ( $i = 0; $i < $_POST['number_fields']; $i=$i+1 )
{
    echo "<tr>";
        //Field name
        echo "<td><input type=text name='field_$i' value=''></td>";

        //Data type
        echo "<td>";
            echo "<select size=1 name='type_$i'>";
                echo "<option value='varchar'>VARCHAR</option>";
                echo "<option value='char'>CHAR</option>";
                echo "<option value='text'>TEXT</option>";
                echo "<option value='tinytext'>TINYTEXT</option>";
                echo "<option value='mediumtext'>MEDIUMTEXT</option>";
                echo "<option value='longtext'>LONGTEXT</option>";
                echo "<option value='date'>DATE</option>";
                echo "<option value='datetime'>DATETIME</option>";
                echo "<option value='timestamp'>TIMESTAMP</option>";
                echo "<option value='time'>TIME</option>";
                echo "<option value='year'>YEAR</option>";
                echo "<option value='int'>INT</option>";
                echo "<option value='tinyint'>TINYINT</option>";
                echo "<option value='smallint'>SMALLINT</option>";
                echo "<option value='mediumint'>MEDIUMINT</option>";
                echo "<option value='bigint'>BIGINT</option>";
                echo "<option value='float'>FLOAT</option>";
                echo "<option value='double'>DOUBLE</option>";
                echo "<option value='decimal'>DECIMAL</option>";
                echo "<option value='blob'>BLOB</option>";
                echo "<option value='tinyblob'>TINYBLOB</option>";
                echo "<option value='mediumblob'>MEDIUMBLOB</option>";
                echo "<option value='longblob'>LONGBLOB</option>";
                echo "<option value='enum'>ENUM</option>";
                echo "<option value='set'>SET</option>";
            echo "</select>";
        echo "</td>";

        //Length/Value
        echo "<td><input type=text name='length_$i' value=''></td>";

        //Null value
        echo "<td>";
            echo "<select size=1 name='null_$i'>";
                echo "<option value='not null'>NOT NULL</option>";
                echo "<option value='null'>NULL</option>";
            echo "</select>";
        echo "</td>";
               
        //Default value
        echo "<td><input type=text name='default_$i' value=''></td>";
               
        //Auto increment
        echo "<td class=TDCenter><input type=checkbox name='auto[]'></td>";
               
        //Primary key
        echo "<td class=TDCenter><input type=radio name='primary_key' value='$i'></td>";

    echo "</tr>";
}
        echo "<tr><td>&nbsp;</td></tr>";
        echo "<tr><td colspan=5 class=TDCenter><input type=submit value='Create table'></td></tr>";
        echo "</table>";
        echo "</form>";
******FORM SCRIPT SLUT*********

I det følgende prøver jeg så at opfange brugerens input, men jeg løber ind i problemer omkring brugen af '$i':

$table_name = $_POST['table_name'];
$number_fields = $_POST['number_fields'];
$primary_key = $_POST['primary_key'];
   
$strSQL = "CREATE TABLE $table_name (";
for ( $i = 0; $i < $number_fields; $i=$i+1 )
{
    $strSQL = $strSQL." ".$_POST['field_$i']." ".$_POST['type_$i'];
    if ($_POST['length_$i'] != "")
    {
        $strSQL = $strSQL."(".$_POST['length_$i'].")";
    }

    if ($_POST['null_$i'] == "not null")
    {
        $strSQL = $strSQL." NOT NULL ";
    }

    if ($_POST['auto[$i]'] == "1")
    {
        $strSQL = $strSQL." AUTO_INCREMENT ";
    }
    $strSQL = $strSQL.", ";
}
$strSQL = $strSQL." PRIMARY KEY($primary_key))";
echo $strSQL;


Eksempel på fejlmeddelelse: Undefined index: type_$i