Using PHP fseek() to Position File Pointers

In PHP file operations, it is often necessary to position the file pointer at a specific location; the built‑in fseek() function provides this capability.

The fseek() function moves the file pointer within an opened file, allowing subsequent read, write, or append actions.

Its syntax is: fseek(file, offset, whence) file (required): the file resource, typically obtained via fopen() .

offset (required): the number of bytes to move; positive moves forward, negative moves backward, zero leaves the pointer unchanged.

whence (optional): determines how the offset is applied; possible values are SEEK_SET (beginning of file, default), SEEK_CUR (current position), and SEEK_END (end of file).

Example usage:

<?php
$file = fopen("example.txt", "r+");
if ($file) {
    // Move pointer to start of file
    fseek($file, 0, SEEK_SET);
    // Read first 10 bytes
    echo fread($file, 10);
    // Move pointer to end of file
    fseek($file, 0, SEEK_END);
    // Write new line
    fwrite($file, "This is a new line.");
    // Move pointer back to start
    fseek($file, 0, SEEK_SET);
    // Read entire file
    echo fread($file, filesize("example.txt"));
    // Close file
    fclose($file);
}
?>

The script opens a file with fopen(), uses fseek() to reposition the pointer, reads and writes data with fread() and fwrite(), and finally closes the file.

Overall, fseek() is a flexible and essential function for precise file manipulation in PHP.