gistfile1.php

<?php

	/**
	 * Slightly modified version of http://www.geekality.net/2011/05/28/php-tail-tackling-large-files/
	 * @author Torleif Berger, Lorenzo Stanco
	 * @link http://stackoverflow.com/a/15025877/995958
	 * @license http://creativecommons.org/licenses/by/3.0/
	 */
	function tailCustom($filepath, $lines = 1, $adaptive = true) {

		// Open file
		$f = @fopen($filepath, "rb");
		if ($f === false) return false;

		// Sets buffer size, according to the number of lines to retrieve.
		// This gives a performance boost when reading a few lines from the file.
		if (!$adaptive) $buffer = 4096;
		else $buffer = ($lines < 2 ? 64 : ($lines < 10 ? 512 : 4096));

		// Jump to last character
		fseek($f, -1, SEEK_END);

		// Read it and adjust line number if necessary
		// (Otherwise the result would be wrong if file doesn't end with a blank line)
		if (fread($f, 1) != "\n") $lines -= 1;
		
		// Start reading
		$output = '';
		$chunk = '';

		// While we would like more
		while (ftell($f) > 0 && $lines >= 0) {

			// Figure out how far back we should jump
			$seek = min(ftell($f), $buffer);

			// Do the jump (backwards, relative to where we are)
			fseek($f, -$seek, SEEK_CUR);

			// Read a chunk and prepend it to our output
			$output = ($chunk = fread($f, $seek)) . $output;

			// Jump back to where we started reading
			fseek($f, -mb_strlen($chunk, '8bit'), SEEK_CUR);

			// Decrease our line counter
			$lines -= substr_count($chunk, "\n");

		}

		// While we have too many lines
		// (Because of buffer size we might have read too many)
		while ($lines++ < 0) {

			// Find first newline and remove all text before that
			$output = substr($output, strpos($output, "\n") + 1);

		}

		// Close file and return
		fclose($f);
		return trim($output);

	}

?>

// Jump back to where we started reading
fseek($f, -mb_strlen($chunk, '8bit'), SEEK_CUR);

Why not use fseek($f, -$seek, SEEK_CUR);