What for? about PHP array and php foreach loop

Question

Possible duplicate:
Why does PHP overwrite values ​​when I repeat this array twice (by reference, by value)

$arr=array('a','b','c');

foreach($arr as &$v){
    echo $v;
}

foreach($arr as $v){
    echo $v;
}

The result is abcabb, not abcabc, why?