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);

	}

?>

Sorry for naive question...
What happens when, during the execution of this code, an attempt is made to write to the log?

Sorry for naive question...
What happens when, during the execution of this code, an attempt is made to write to the log?

you must open the file in LOCK exclusive mode then or make a copy (if it's possible)

https://www.w3schools.com/php/func_filesystem_flock.asp

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

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