This is listed in the documentation above, but it's a bit tucked away between the paragraphs. The difference between a series of if statements and the switch statement is that the expression you're comparing with, is evaluated only once in a switch statement. I think this fact needs a little bit more attention, so here's an example:

<?php
$a = 0;

if(++$a == 3) echo 3;
elseif(++$a == 2) echo 2;
elseif(++$a == 1) echo 1;
else echo "No match!";

// Outputs: 2

$a = 0;

switch(++$a) {
    case 3: echo 3; break;
    case 2: echo 2; break;
    case 1: echo 1; break;
    default: echo "No match!"; break;
}

// Outputs: 1
?>

It is therefore perfectly safe to do:

<?php
switch(winNobelPrizeStartingFromBirth()) {
case "peace": echo "You won the Nobel Peace Prize!"; break;
case "physics": echo "You won the Nobel Prize in Physics!"; break;
case "chemistry": echo "You won the Nobel Prize in Chemistry!"; break;
case "medicine": echo "You won the Nobel Prize in Medicine!"; break;
case "literature": echo "You won the Nobel Prize in Literature!"; break;
default: echo "You bought a rusty iron medal from a shady guy who insists it's a Nobel Prize..."; break;
}
?>

without having to worry about the function being re-evaluated for every case. There's no need to preemptively save the result in a variable either.