$km = 5

 if ($km<3) { $tri = 100; }
if ($km<=6 ) { $sest = 80; }
if ($km<=10) { $deset = 70; }
if ($km>10) { $vic= 60; }

switch ($km)
{case "$tri"; echo $km * $tri break;
 case "$sest" ; echo $km * $sest; break;
 case "$deset"; echo $km * $deset; break;
 case "$vic"; echo $km * $vic; break;
 default; echo "napiš počet km"; }