Php keep checkboxes checked after form error - checkbox

Question

How can i keep the selected checkboxes checked, after the form submit on error? Now, i get this error message: Warning: in_array() expects parameter 2 to be array, string given in
Thanks!
function runsql_array($sql)
{
global $kapcs;
$res = mysqli_query($kapcs , $sql) or die(mysqli_error( $kapcs));
if (mysqli_num_rows($res) == 0)
{
return array();
}
else
{
$out = array();
while ($a = mysqli_fetch_assoc($res))
{
$out[] = $a;
}
return $out;
}
}
<td>
<?php
$ertek = isset($_POST["termek_tul_tipusok"]) ? $_POST["termek_tul_tipusok"] : '' ;
$values = runsql_array("SELECT termek_tipus_id, termek_tipus_nev FROM termek_tipusok WHERE termek_tipus_status = 1
ORDER BY termek_tipus_nev ASC");
foreach($values as $val=>$szoveg)
{
$checked = in_array($val, $ertek) ? ' checked ' : '' ;
echo '<div style="margin:4px 0;"><label style="cursor:pointer;" for="tulajdonsag-'.$val.'">';
echo '<input id="tulajdonsag-'.$val.'" type="checkbox" name="termek_tul_tipusok[]" '.$checked.' value="'.$val.'" />';
echo $szoveg['termek_tipus_nev'];
echo '</label></div>';
}
?>
</td>

Answer 1

the line
$ertek = isset($_POST["termek_tul_tipusok"]) ? $_POST["termek_tul_tipusok"] : '' ;
defines $ertek. And in the case that the post dosnt exist its defined as an empty string. You should use an empty Array.
$ertek = isset($_POST["termek_tul_tipusok"]) ? $_POST["termek_tul_tipusok"] : array() ;