SeekableFileReader.php

<?php
class SeekableFileReader {
    
    /* указатель на файл */
    private $handle;
    /* длина файла (кол-во строк)*/
    private $fileLength = 0;
    /* массив позиций для ф-и fseek() */
    private $seekArr = [];
    /* текущая позиция */
    private $position = 0;

    function __construct($filename) {
        $this->handle = fopen($filename, "r");
        $this->getFileLength();
    }
    
    function __destruct() {
        $this->close();
    }

    /* вычисляет длинну файла */
    private function getFileLength() {
        $this->seekArr[] = ftell($this->handle);
         while (fgets($this->handle) !== false) { 
             /* записывам указатель на позицию */          
            $this->seekArr[] = ftell($this->handle);
            /* записываем длинну */
            $this->fileLength++;
        }
    }

    /* считывает строку из текстового файла */
    private function getRow() {
        if(!feof($this->handle)) {
            return trim(fgets($this->handle));
        } else {            
            return false;
        }
    }

    public function getSeekArr(){
        return $this->seekArr;
    }  

    public function close(){
        fclose($this->handle);
    }


    public function seek($position)
    {
        if (!$this->valid($position)) {
            throw new \OutOfBoundsException("invalid seek position ($position)");
        }
        $this->position = $position;
    }

    public function rewind()
    {
        $this->position = 0;
    }

    public function current()
    {        
        fseek($this->handle, $this->seekArr[$this->position]);
        return $this->getRow();            
    }

    public function key()
    {
        return $this->position;
    }

    public function next()
    {
        if($this->position < $this->fileLength-1) $this->position++;        
    }

    public function prev()
    {
        if($this->position != 0) $this->position--;
    }

    public function valid($position)
    {
        return (($position < $this->fileLength) && ($position >= 0));
    }

}

/* tests */

$file = new SeekableFileReader('./package.json');

print_r($file->current()."\r\n");
$file->next();
$file->next();
$file->next();
$file->next();
$file->next();
print_r($file->current()."\r\n");
$file->prev();
$file->prev();
$file->prev();
$file->prev();
$file->prev();
$file->prev();
$file->prev();
print_r($file->current()."\r\n");

$file->seek(1000);