//mysql_baglantısı_yap...

<?php 

if ($_POST){

    $kullanici_adi=mysql_real_escape_string($_POST['veri']);

    mysql_query("INSERT INTO gorev (kullanici) values ('$kullanici_adi')");


}else{

    $sql=mysql_query("SELECT * FROM gorev WHERE kullanici='Ahmet' OR kullanici='Mehmet' ");

    $data=array();

    if (mysql_num_rows($sql) > 0){
        while($row=mysql_fetch_assoc($sql)){
            $data[]=$row['kullanici'];
        }        
    }

?>

<form action="" method="post">
    <input type="submit" name="veri" value="Ahmet" <?php if (in_array('Ahmet',$data)){ }else{ echo 'disabled="disabled"'; } ?> />
    <input type="submit" name="veri" value="Mehmet" <?php if (in_array('Mehmet',$data)){ }else{ echo 'disabled="disabled"'; } ?> />
</form>
<?php
}

?>