This function will read $file backwards, $blocksize bytes at a time, until it finds that it contains at least $lines "\n" characters. It then breaks the buffer into an array of lines, pares out the extras, and returns the data according to the optional arguments $desired_endl and $as_array.

get_last_lines.php

<?php
function get_last_lines($file, $lines, $as_array=false, $desired_endl="\n", $blocksize=1024) {
    $buffer = '';
    if( ! $fh = fopen($file, 'r') ) { die('could not open file'); }

    fseek($fh, ($blocksize * -1) - 1, SEEK_END);
    while(true) {
        $buffer = fread($fh, $blocksize) . $buffer;
        if( preg_match_all("/\n/", $buffer, $trash) > $lines) {
            break;
        }
        $curpos = ftell($fh);
        if( $curpos - $blocksize * 2 <= 0 ) {
            rewind($fh);
            $buffer = fread($fh, $blocksize - $curpos) . $buffer;
            break;
        }
        fseek($fh, $curpos - $blocksize * 2);
    }

    $b_arr = array_map(function($a){
            return preg_replace("/\r\$/", "", $a);
        }, explode("\n", $buffer));

    $b_arr = array_reverse(array_slice(array_reverse($b_arr), 0, $lines));
    if( $as_array ) { return $b_arr; }
    else { return implode($desired_endl, $b_arr); }
}

echo get_last_lines('/var/log/messages', 20, false, "\n", 32);