why are assignments in conditions bad?

if (($row = $db->get_row($sql))) {
        return $row->folder;
    } else {
        return FALSE;
    }

if (a = 1) { .. }

if (a == 1) { .. }

if (1 == a) { .. }

if (a = 1) { .. }

if(a = b)
   //logic error

if ( a=func(x) && b=func(y) )
{
  // do this
}

if ( a==func(x) && b==func(y) ) ...

$counter = 0;
while( $getWhateverDataObj = mysql_fetch_object( $sqlResult )) {
   $getWhateverObj->firstName[$counter] = $getWhateverDataObj->firstName;
   $getWhateverObj->lastName[$counter]  = $getWhateverDataObj->lastName;
   $counter++;
}

$next = mysql_fetch_assoc($result)
do{
...
...
...

$next = mysql_fetch_assoc($result) or break;
}while ($next)