ServeFilePartial.inc.php

<?php

function serveFilePartial($fileName, $fileTitle = null, $contentType = 'application/octet-stream')
{
	if( !file_exists($fileName) )
		throw New \Exception(sprintf('File not found: %s', $fileName));

	if( !is_readable($fileName) )
		throw New \Exception(sprintf('File not readable: %s', $fileName));


	### Remove headers that might unnecessarily clutter up the output
	header_remove('Cache-Control');
	header_remove('Pragma');


	### Default to send entire file
	$byteOffset = 0;
	$byteLength = $fileSize = filesize($fileName);

	header('Accept-Ranges: bytes', true);
	header(sprintf('Content-Type: %s', $contentType), true);

	if( $fileTitle )
		header(sprintf('Content-Disposition: attachment; filename="%s"', $fileTitle));

	### Parse Content-Range header for byte offsets, looks like "bytes=11525-" OR "bytes=11525-12451"
	if( isset($_SERVER['HTTP_RANGE']) && preg_match('%bytes=(\d+)-(\d+)?%i', $_SERVER['HTTP_RANGE'], $match) )
	{
		### Offset signifies where we should begin to read the file
		$byteOffset = (int)$match[1];


		### Length is for how long we should read the file according to the browser, and can never go beyond the file size
		if( isset($match[2]) ){
			$finishBytes = (int)$match[2];
	        	$byteLength = $finishBytes + 1;
		} else {
			$finishBytes = $fileSize - 1;
		}
	
		$cr_header = sprintf('Content-Range: bytes %d-%d/%d', $byteOffset, $finishBytes, $fileSize);
	
		header("HTTP/1.1 206 Partial content");
		header($cr_header);  ### Decrease by 1 on byte-length since this definition is zero-based index of bytes being sent
	}

	$byteRange = $byteLength - $byteOffset;

	header(sprintf('Content-Length: %d', $byteRange));

	header(sprintf('Expires: %s', date('D, d M Y H:i:s', time() + 60*60*24*90) . ' GMT'));


	$buffer = ''; 	### Variable containing the buffer
	$bufferSize = 512 * 16; ### Just a reasonable buffer size
	$bytePool = $byteRange; ### Contains how much is left to read of the byteRange

	if( !$handle = fopen($fileName, 'r') )
		throw New \Exception(sprintf("Could not get handle for file %s", $fileName));

	if( fseek($handle, $byteOffset, SEEK_SET) == -1 )
		throw New \Exception(sprintf("Could not seek to byte offset %d", $byteOffset));


	while( $bytePool > 0 )
	{
		$chunkSizeRequested = min($bufferSize, $bytePool); ### How many bytes we request on this iteration

		### Try readin $chunkSizeRequested bytes from $handle and put data in $buffer
		$buffer = fread($handle, $chunkSizeRequested);

		### Store how many bytes were actually read
		$chunkSizeActual = strlen($buffer);

		### If we didn't get any bytes that means something unexpected has happened since $bytePool should be zero already
		if( $chunkSizeActual == 0 )
		{
			### For production servers this should go in your php error log, since it will break the output
			trigger_error('Chunksize became 0', E_USER_WARNING);
			break;
		}

		### Decrease byte pool with amount of bytes that were read during this iteration
		$bytePool -= $chunkSizeActual;

		### Write the buffer to output
		print $buffer;

		### Try to output the data to the client immediately
		flush();
	}

	exit();
}