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