Consuming variable lengths of data from a fixed length buffer in PHP

I have a file that's so large I'm unable to read it into a string in one go, but have to use buffering:

$fp = @fopen("bigfile", 'rb');
while (!feof($fp)) {
    //process buffer
}

For simplicity, say the file contains a sequence of integer string pairs, where the integer holds the length of the string. Then the code I want to realise in process buffer, is unpack an int, read that many characters from the buffer, then repeat.

I appreciate any suggestions in dealing with the scenario where the string spans one buffer to the next. I'm sure that this problem must have been solved and that there is a design pattern for it, I just don't know where to start looking.

Any help would be appreciated.