Create joined string by looping through key value array

I am trying to use a variable in a foreach loop but I am getting strange results. The first foreach loop work fine but gives a notice of a undefined variable and in the second version there is no notice but it only returns the last item in the array.

$formats = array(
    'application/x-mpegurl' => 'hls',
    'video/webm'            => 'webm',
    'video/mp4'             => 'mp4',
    'video/ogg'             => 'ogg',
    'video/flash'           => 'flash',
);

// Works perfectly but there a undefined variable $source
foreach( $formats as $format => $src ){
    if ( !empty( $src ) ) {
        $source .= '<source type="' . $format . '" src="' . $src . '">';
    }
}
echo $source;

// Returns only the last item in the variable but there is no undefined variable
foreach( $formats as $format => $src ){
    $source2 = '';
    if ( !empty( $src ) ) {
        $source2 .= '<source type="' . $format . '" src="' . $src . '">';
    }
}
echo $source2;

I have googled for solutions have not found any.

Define $source an d $source1 before the loops start.

 $source = "";
 // loop starts here

Complete code:

$source = "";
foreach( $formats as $format => $src ){
   if ( !empty( $src ) ) {
     $source .= '<source type="' . $format . '" src="' . $src . '">';
   }
}
echo $source;

$source2 = '';
foreach( $formats as $format => $src ){
   if ( !empty( $src ) ) {
      $source2 .= '<source type="' . $format . '" src="' . $src . '">';
   }
}
echo $source2;