PHP pack binary data

Question

This is with regard to data sent over a socket to a C application residing on a remote POS system.

Binary data is sent from a php application, in the C app packet structure there is 64bytes stored for a string eg a product name.

Now when I send the product name across network through php sockets, I use pack to convert data to binary

$value = 'product name' 
$qty = 2;
$len = strlen($value);
$output = '';
for($i=0; $i<$len; $i++) {
        $output .= pack('c', ord(substr($value, $i, 1))).pack('c',$qty) 
}

When the data is received by the C application the string contains a lot of garbage data, including numbers and special characters.

Which of the pack options i have to use to pack the product name into a 64byte binary string that will be interpreted by the C application in the correct format.

Answer 1

Sending binary data through network sockets also may create troubles with byte ordering (Endianess), you might wanna check out if the byte ordering is the same on both systems. http://en.wikipedia.org/wiki/Endianness

Answer 2

Your loop to build $output produces a string like "p\\x02r\\x02o\\x02d\\x02u\\x02c\\x02t\\x02 \\x02n\\x02a\\x02m\\x02e\\x02"

If your C program expects something like "product name\\0\\x02" , then the loop should be:

$output = '';
for($i=0; $i<$len; $i++) {
        $output .= pack('c', ord(substr($value, $i, 1))); 
}
$output .= pack('c',0).pack('c',$qty);