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

fseek()

moves the file pointer to a specified location, enabling reading, writing, or appending at arbitrary positions within an opened file.

Syntax of fseek()

fseek(file, offset, whence)

file

: required; the file resource opened by fopen(). offset: required; the number of bytes to move the pointer. Positive values move forward, negative values move backward, zero leaves the pointer unchanged. 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 start of the file
    fseek($file, 0, SEEK_SET);

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

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

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

    // Move pointer back to the start
    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 full contents before closing the file.

Through this example, it is clear that fseek() is a flexible and essential function for precise file handling in PHP, allowing developers to add, delete, or modify data at any location within a file.