ช่วยอธิบาย หลักการทำงานของ switch case ให้หน่อยครับ

switch ($day) { 
   case 1 : 
      echo "Monday<BR>\n"; 
      break; 
case 2 : 
      echo "Tuesday<BR>\n"; 
      break; 
   case 3 : 
      echo "Wednesday<BR>\n"; 
      break; 
   case 4 : 
      echo "Thurday<BR>\n"; 
      break; 
   case 5 : 
      echo "Friday<BR>\n"; 
      break; 
   case 6: 
      echo "Saturday<BR>\n"; 
      break; 
   case 7 : 
      echo "Sunday<BR>\n"; 
      break; 
   default : 
      echo "error<BR>\n"; 
}

switch ($answer) { 
   case "yes" : 
      echo "The user said 'yes'.\n"; 
      break; 
   case "no" : 
      echo "The user said 'no'.\n"; 
      break; 
   default: 
      echo "The user said neither 'yes' nor 'no'.\n"; 
}

switch ($answer) { 
   case "yes" : 
   case "no" : 
      echo "The user said '",$answer,"'.\n"; 
      break; 
   default: 
      echo "The user said neither 'yes' nor 'no'.\n"; 
}