Report weird PHP bug in foreach loop?

Question

I hope the following code is self-explanatory.

The original code was taken from a user contribution in php.net/manual...str-getcsv. It took me near half a day of work to finally figure out where the bug was generated.

It is completely reproducible.

The bug:

// THE BUG
$arr = array( "a", "b", "c", "d" );
foreach ( $arr as &$elem ) $elem = $elem . "1"; // bug origin
print_r( $arr );
echo "<pre>";
foreach ( $arr as $key => $elem ) echo "elem {$key} => {$elem}\n" ;
echo "</pre>";

Output:

Array
    (
        [0] => a1
        [1] => b1
        [2] => c1
        [3] => d1  <--- notice this
    )
elem 0 => a1
elem 1 => b1
elem 2 => c1
elem 3 => c1  <--- bug result !!!????

Workaround

// WORKAROUND 
$arr = array( "a", "b", "c", "d" );
foreach ( $arr as $key => $elem ) $arr[$key] = $elem . "1"; // alternative code
print_r( $arr );
echo "<pre>";
foreach ( $arr as $key => $elem ) echo "elem {$key} => {$elem}\n" ;
echo "</pre>";

Output:

Array
    (
        [0] => a1
        [1] => b1
        [2] => c1
        [3] => d1
    )
elem 0 => a1
elem 1 => b1
elem 2 => c1
elem 3 => d1  <--- no bug