Master PHP File Positioning with fseek(): Syntax, Options, and Examples

In PHP file operations, you often need to position the file pointer at a specific location. PHP provides the useful function fseek() for this purpose.

The fseek() function moves the file pointer to a specified position, allowing reading, writing, or appending data.

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 stays.

whence : optional. Determines how the offset is applied. Possible values:

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 first 10 bytes
    echo fread($file, 10);

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

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

    // Move pointer back to the beginning
    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() in read‑write mode, uses fseek() to reposition the pointer, reads and writes data with fread() and fwrite(), and finally closes the file.

Overall, fseek() is a flexible function that enables locating any position within a file, facilitating operations such as adding, deleting, or modifying data efficiently.