Mastering PHP’s fseek(): How to Move File Pointers Efficiently

fseek()

function moves the file pointer to a specified location, enabling reading, writing, or appending at different positions in an opened file.

Syntax of fseek()

fseek(file, offset, whence)

file

: required, the file resource opened with fopen(). offset: required, number of bytes to move the pointer; positive moves forward, negative moves backward, zero keeps the current position. whence: optional, determines how the offset is interpreted. Possible values are: SEEK_SET (default): set pointer to the beginning of the file. SEEK_CUR: set pointer relative to the current position. SEEK_END: set pointer relative to the end of the file.

Example usage of fseek()

<?php
$file = fopen("example.txt", "r+");
if ($file) {
    // Move pointer to the beginning
    fseek($file, 0, SEEK_SET);

    // Read and output the first 10 bytes
    echo fread($file, 10);

    // Move pointer to the end
    fseek($file, 0, SEEK_END);

    // Write a new line to the file
    fwrite($file, "This is a new line.");

    // Move pointer back to the beginning
    fseek($file, 0, SEEK_SET);

    // Read and output the entire file
    echo fread($file, filesize("example.txt"));

    // Close the file
    fclose($file);
}
?>

The script opens a file in read‑write mode, uses fseek() to position the pointer at the start, reads the first 10 bytes, moves to the end to append a new line, then returns to the start to read the whole file before closing it.

Overall, fseek() is a flexible and essential function for precise file manipulation in PHP, allowing developers to add, delete, or modify data at any location within a file.