PHP interpret a 4 byte binary string as an unsigned big endian integer

Question

I am trying to interpret a binary string as an unsigned big endian integer, as her instructions here: http://mimesniff.spec.whatwg.org/#matches-the-signature-for-mp4 (point 4)

I'm not quite sure what I need to do here, but here are my attempts:

// ONE
$box_size   = substr( $sequence, 0, 4 );
$box_size   = pack( 'C*', $box_size[0], $box_size[1], $box_size[2], $box_size[3] );
$box_size   = unpack( 'N*', $box_size );

// TWO
$box_size   = substr( $sequence, 0, 4 );
$box_size   = array_map( 'ord', str_split( $box_size ) );

// THREE
$box_size   = substr( $sequence, 0, 4 );
$box_size   = bindec( $box_size );

// FOUR
$box_size   = substr( $sequence, 0, 4);
$box_size   = (int) $box_size;

I have had no luck, and honestly am not sure what the result should even be.. Does anyone understand this? I think I might be on the right track with pack and unpack.

Wow... That was very easy. The difference between pack and unpack confuses me.. But this seems to have worked a treat. Thanks deceze! — Shane
– Shane, Commented Apr 19, 2013 at 2:07

deceze · Accepted Answer · 2013-04-19 03:00:10Z

4

I'll go ahead and post the comment as an answer then...

To unpack a "compact" bit representation of something into a native type, just use unpack with the right parameters to denote the type of bits you're unpacking. In your case:

$unpacked = unpack('N', $unsignedBigEndianInteger);
$int = $unpacked[1];

This makes PHP read the byte stream assuming it represents an unsigned big endian long and convert it into a PHP native integer.

Remember:

pack: "large native type" → squeeze into byte representation
unpack: compact byte representation → "large native type"

edited Apr 19, 2013 at 3:00

answered Apr 19, 2013 at 2:14

deceze♦

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1 Comment

Shane Over a year ago

Sorry, shouldn't it be $unpacked[1]; ? Thanks for that little end note :)

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