Switch too often triggered - php

Question

I cannot get my head around this. There might be a simple solution and the problem addressed already, but I could not find an answer.
<?php
$string = '';
$array = array(1, 2, 3);
foreach($array as $number) {
switch($number) {
case '1':
$string .= 'I ';
case '2':
$string .= 'love ';
case '3':
$string .= 'you';
}
}
echo $string;
?>
As you might have guessed, the sentence produced should be: I love you
But that is the actual output: I love youlove youyou
How is this even possible, when the switch is only triggered thrice.
Meanwhile I know that the problem can be fixed with a >break;< after each case. But I still do not understand why this is necessary.
I would be very happy, if someone could explain to me what PHP is doing.
Happy Valentines!

Answer 1

When case 1 is matched it will also execute case 2 and 3 unless there is a break.
So each time through the loop the matched case and everything following is executed.
First time: case 1, 2, 3
Second time: case 2, 3
Third time: case 3

Answer 2

For reference, here's an extract from the PHP documentation on switch, which explains it better than I probably would. :)
It is important to understand how the switch statement is executed in order to avoid mistakes. The switch statement executes line by line (actually, statement by statement). In the beginning, no code is executed. Only when a case statement is found with a value that matches the value of the switch expression does PHP begin to execute the statements. PHP continues to execute the statements until the end of the switch block, or the first time it sees a break statement. If you don't write a break statement at the end of a case's statement list, PHP will go on executing the statements of the following case. For example:
<?php
switch ($i) {
case 0:
echo "i equals 0";
case 1:
echo "i equals 1";
case 2:
echo "i equals 2";
}
?>
Here, if $i is equal to 0, PHP would execute all of the echo statements! If $i is equal to 1, PHP would execute the last two echo statements. You would get the expected behavior ('i equals 2' would be displayed) only if $i is equal to 2. Thus, it is important not to forget break statements (even though you may want to avoid supplying them on purpose under certain circumstances).

Answer 3

When you don't specify a break the code execution will fall-through to the next case.
As an example an exam paper with a score out of 10 could be graded as such:
switch ($score)
{
case 10:
// A+ when score is 10
echo 'A+';
break;
case 9:
case 8:
// A when score is 8 or 9
echo 'A';
break;
case 7:
case 6:
// B when score is 7 or 6
echo 'B';
break;
case 5:
case 4:
case 3:
// C when score between 3 and 5
echo 'C';
break;
default:
// Failed if score is less than 3
echo 'Failed';
}