PHP fseek Function

last modified April 3, 2025

The PHP fseek function sets the file position indicator for a file pointer. It allows random access to file contents by moving the pointer.

Basic Definition

The fseek function moves the file pointer to a specified position. It takes three parameters: the file pointer, offset, and optional whence.

Syntax: fseek(resource $stream, int $offset, int $whence = SEEK_SET): int. Returns 0 on success, -1 on failure. Works with files opened in binary mode.

Basic fseek Example

This shows the simplest usage of fseek to move the file pointer.

basic_fseek.php

<?php

declare(strict_types=1);

$file = fopen(‘data.txt’, ‘rb’); if ($file === false) { die(“Failed to open file”); }

fseek($file, 10); echo fread($file, 20);

fclose($file);

This moves the pointer to position 10 and reads 20 bytes. The default whence parameter is SEEK_SET, meaning offset is from the start of the file.

Seeking from Current Position

Using SEEK_CUR moves the pointer relative to its current position.

fseek_current.php

<?php

declare(strict_types=1);

$file = fopen(‘data.txt’, ‘rb’); if ($file === false) { die(“Failed to open file”); }

fseek($file, 10, SEEK_SET); // Move to position 10 fseek($file, 5, SEEK_CUR); // Move 5 more bytes echo fread($file, 15);

fclose($file);

This first moves to position 10, then moves 5 more bytes forward. The final position is 15 bytes from the start of the file.

Seeking from End of File

Using SEEK_END moves the pointer relative to the end of the file.

fseek_end.php

<?php

declare(strict_types=1);

$file = fopen(‘data.txt’, ‘rb’); if ($file === false) { die(“Failed to open file”); }

fseek($file, -10, SEEK_END); // 10 bytes from end echo fread($file, 10);

fclose($file);

This positions the pointer 10 bytes before the end of the file. Negative offsets are often used with SEEK_END to read the end of files.

Checking File Position

Combine fseek with ftell to verify positions.

fseek_ftell.php

<?php

declare(strict_types=1);

$file = fopen(‘data.txt’, ‘rb’); if ($file === false) { die(“Failed to open file”); }

fseek($file, 25); echo “Position: " . ftell($file) . “\n”;

fseek($file, -5, SEEK_CUR); echo “New position: " . ftell($file) . “\n”;

fclose($file);

This shows the current position before and after seeking. ftell returns the current position measured in bytes from the file start.

Error Handling

Check fseek return value for error handling.

fseek_error.php

<?php

declare(strict_types=1);

$file = fopen(‘data.txt’, ‘rb’); if ($file === false) { die(“Failed to open file”); }

$result = fseek($file, 1000000); if ($result === -1) { echo “Seek failed - position may be beyond EOF\n”; } else { echo “Seek successful\n”; }

fclose($file);

This attempts to seek beyond likely file size. The function returns -1 on failure. Note that seeking beyond EOF isn’t always an error in PHP.

Best Practices

• Binary Mode: Always open files in binary mode for reliable seeking.

• Error Checking: Verify both fopen and fseek return values.

• Position Validation: Check positions don’t exceed file size.

• Resource Cleanup: Always close files with fclose.

• Large Files: Use with caution on very large files (>2GB).

Source

PHP fseek Documentation

This tutorial covered the PHP fseek function with practical examples showing its usage in different scenarios.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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