Pure php implementation of tail -f

phptailf.php

<?php

/**
 * Tailf
 * A pure PHP implementation of the linux command tail -f
 *
 * 20250722 gbh
 */

// duration for usleep
define('ONE_TENTH_SECOND', 100000);

// number of lines to output on tail
define('TAIL_LINE_COUNT', 10);

/**
 * Entry point
 */
tailf(filePath($argv));

/**
 * Gets file path from cli arguments, tests for validity
 * and returns it.
 * Dies with message on any fail.
 * 
 * @param array<string> $argv
 * @return string
 */
function filePath($argv): string {
    $file = $argv[1] ?? null;
    if(!$file) {
        die("Must pass a file as argument");
    }
    if(!file_exists($file)) {
        die("No file at '$file'");
    }
    if(!is_readable($file)) {
        die("Cannot read file at '$file'");
    }
    return $file;
}

/**
 * Opens a file, returns the pointer resource
 *
 * @param  string $file Path to file
 * @return mixed The file pointer resource
 */
function open(string $file): mixed {
    $fp = fopen($file, "r");
    return $fp;
}

/**
 * Updates a file pointer $fp to be n bytes from the end of the file
 * so that outputting to eof prints the last $lineCount lines.
 *
 * @param  mixed $fp The file pointer resource
 * @param  int $lineCount How many lines back from the end of file
 * @return mixed The file pointer resource
 */
function windToTailStart(mixed $fp, int $lineCount): mixed {
    fseek($fp, 0, SEEK_END);
    $position = -1;
    $lineCounter = 0;
    do {

		if(ftell($fp) == 0) {
			return $fp;
		}

        fseek($fp, $position--, SEEK_END);
        if(fgetc($fp) == PHP_EOL) {
            $lineCounter++;
        }
    }
    while($lineCounter < $lineCount);
    return $fp;
}

/**
 * Outputs all lines of file from the position of the 
 * file pointer $fp to the end of file.
 *
 * @param  mixed $fp The file pointer resource
 * @return void
 */
function output(mixed $fp) {
    while(!feof($fp)) {
        print fgets($fp);
    }
}

/**
 * Outputs the last $lineCount lines from file $file.
 *
 * @param  string $file Path to file
 * @param  int $lineCount
 * @return void
 */
function tail(string $file, int $lineCount) {
	$fp = open($file);
    $fp = windToTailStart($fp, $lineCount);
    output($fp);
    fclose($fp);
}

/**
 * Outputs the last $lineCount lines from file $file
 * then waits for the file to update and outputs the 
 * new lines. ie. `tail -f`
 *
 * @param  string $file Path to file
 * @param  int $lineCount
 * @return void
 */
function tailf(string $file, int $lineCount = TAIL_LINE_COUNT) {
    // output tail content
    tail($file, $lineCount);

    // open file at the end
    $fp = open($file);
    fseek($fp, 0, SEEK_END);

    // wait for new file content and output
    while(true) { /** @phpstan-ignore while.alwaysTrue */
        $tell = ftell($fp);
        usleep(ONE_TENTH_SECOND);
        fseek($fp, 0, SEEK_END);
        if($tell != ftell($fp)) {
            fseek($fp, $tell, SEEK_SET);
            output($fp);
        }
    }
}