h2utils.php

<?php

function binToInt16(string $bytes): int
{
    return (ord($bytes[0]) << 8) | ord($bytes[1]);
}

function binToInt24(string $bytes): int
{
    return (ord($bytes[0]) << 16) |
           (ord($bytes[1]) << 8) |
            ord($bytes[2]);
}

function binToInt32(string $bytes): int
{
    return (ord($bytes[0]) << 24) |
           (ord($bytes[1]) << 16) |
           (ord($bytes[2]) <<  8) |
            ord($bytes[3]);
}

function int16ToBin(int $value): string
{
    return chr(($value >> 8) & 0xff).chr($value & 0xff);
}

function int24ToBin(int $value): string
{
    return chr(($value >> 16) & 0xff).
           chr(($value >>  8) & 0xff).
           chr($value & 0xff);
}

function int32ToBin(int $value): string
{
    return chr(($value >> 24) & 0xff).
           chr(($value >> 16) & 0xff).
           chr(($value >>  8) & 0xff).
           chr($value & 0xff);
}

function createFrameHeader(int $length, int $type, int $flag, int $streamId): string
{
    return int24ToBin($length).chr($type).chr($flag).int32ToBin($streamId);
}

function frameHeaderToArray(string $bytes): array
{
    return [
        'length' => substr($bytes, 0, 3),
        'type' => substr($bytes, 3, 1),
        'flag' => substr($bytes, 4, 1),
        'streamId' => substr($bytes, 5, 4)
    ];
}

function frameToArray(string $bytes): array
{
    $ret = frameHeaderToArray($bytes);
    $ret['payload'] = substr($bytes, 9);
    return $ret;
}

test.php

<?php

require 'h2utils.php';

var_dump(
    0x0 === binToInt24("\x00\x00\x00"),
    0x0 === binToInt32("\x00\x00\x00\x00"),
    "\x00\x00\x00" === int24ToBin(0x0),
    "\x00\x00\x00\x00"=== int32ToBin(0x0)
);

$settings = implode([
    "length" =>  "\x00\x00\x00",
    "type" => "\x04",
    "flag" => "\x00",
    "streamId" => "\x00\x00\x00\x00",
    "payload" => ""
]);

$settings2 = implode([
   "length" => "\x00\x00\x0C",
   "type"   => "\x04",
   "flag" => "\x00",
   "streamId" => "\x00\x00\x00\x00",
   "payload" => "\x00\x02"."\x00\x00\x00\x00".
                "\x00\x03"."\x00\x00\x00\x64"
]);

var_dump(
    $settings === createFrameHeader(0x0, 0x4, 0x0, 0x0),
    $settings === implode(frameHeaderToArray($settings)),
    $settings2 === implode(frameToArray($settings2))
);