CODEWITHSUNDEEP
phpstringbit-manipulation
donatJ
donatJ

Reputation: 3374

Convert Int into 4 Byte String in PHP

I need to convert an unsigned integer into a 4 byte string to send on a socket.

I have the following code and it works, but it feels... disgusting.

/**
 * @param $int
 * @return string
 */
 function intToFourByteString( $int ) {
    $four  = floor($int / pow(2, 24));
    $int   = $int - ($four * pow(2, 24));
    $three = floor($int / pow(2, 16));
    $int   = $int - ($three * pow(2, 16));
    $two   = floor($int / pow(2, 8));
    $int   = $int - ($two * pow(2, 8));
    $one   = $int;

    return chr($four) . chr($three) . chr($two) . chr($one);
}

My friend who uses C says I should be able to do this with bitshifts but I don't know how and he isn't familiar enough with PHP to be helpful. Any help would be appreciated.

To do the reverse I already have the following code

/**
 * @param $string
 * @return int
 */
function fourByteStringToInt( $string ) {
    if( strlen($string) != 4 ) {
        throw new \InvalidArgumentException('String to parse must be 4 bytes exactly');
    }

    return (ord($string[0]) << 24) + (ord($string[1]) << 16) + (ord($string[2]) << 8) + ord($string[3]);
}

Upvotes: 2

Views: 3927

Answers (1)

georg
georg

Reputation: 214959

This is actually as simple as

$str = pack('N', $int);

see pack. And the reverse:

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

If you're curious how to do packing using bit shifts, it goes like this:

function intToFourByteString( $int ) {
    return
        chr($int >> 24 & 0xFF).
        chr($int >> 16 & 0xFF).
        chr($int >>  8 & 0xFF).
        chr($int >>  0 & 0xFF);
}

Basically, shift eight bits each time and mask with 0xFF (=255) to remove high-order bits.

Upvotes: 5

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