INSERT not working when selection value is -1 - php

Question

I have problem, that if I not select any item from selection list, they not inserting element in my DB.
I put some come, that if I select value -1 from list, which is ------------, then they should make it NULL in that field.
There is my code.
<p>
<?php
if ($_POST['fk_KOMANDAid_KOMANDA'] == '-1') {
$_POST['fk_KOMANDAid_KOMANDA'] = NULL;}
?>
<label class="field" for="fk_KOMANDAid_KOMANDA">Komanda<?php echo in_array('fk_KOMANDAid_KOMANDA', $required) ? '<span> *</span>' : ''; ?></label>
<select id="fk_KOMANDAid_KOMANDA" name="fk_KOMANDAid_KOMANDA">
<option value="-1">---------------</option>
<?php
$kom = $asmuoObj->getkomanda();
foreach($kom as $key => $val) {
$selected = "";
if(isset($data['fk_KOMANDAid_KOMANDA']) && $data['fk_KOMANDAid_KOMANDA'] == $val['id']) {
$selected = " selected='selected'";}
echo "<option{$selected} value='{$val['id']}'>{$val['pavadinimas']}</option>";}
?>
</select>
</p>
This is INSERT function.
public function insertkomanda($data) {
$query = " INSERT INTO {$this->komanda_lentele}
(
`pavadinimas`,
`sutrumpinimas`,
`ikurimo_data`,
`svetaines_adresas`,
`fk_VALSTYBEid_VALSTYBE`,
`fk_RUNGTYNESid_RUNGTYNES`
)
VALUES
(
'{$data['pavadinimas']}',
'{$data['sutrumpinimas']}',
'{$data['ikurimo_data']}',
'{$data['svetaines_adresas']}',
'{$data['fk_VALSTYBEid_VALSTYBE']}',
'{$data['fk_RUNGTYNESid_RUNGTYNES']}'
)";
mysql::query($query);
}
When I press SUBMIT, then nothing happens, data not intserted in my DB. Should insert with fk_KOMANDAid_KOMANDA eaquls NULL.