Randomize weighted array with weights lower than 1 and decimals

I have an array with weights on each value used to retrieve a random value based on those weights. I have used something like this in the past:

$items = array( "value1" => 30, "value2" => 70 );

$weighted = array();

foreach( $items as $value => $weight ) {
    $weighted = array_merge($weighted, array_fill(0, $weight, $value));
}

echo $result = $weighted[array_rand($weighted)];

But what if I need to use decimals on the weights? (The weights I use always sum up to 100 btw)

For example:

$items = array( "value1" => 0.5, "value2" => 99.5 );

So value1 would only be shown 0.5% of the times.