PHP fwrite Function
last modified April 3, 2025
The PHP fwrite function writes data to a file. It's a fundamental
function for file operations in PHP. This tutorial covers its usage in depth.
Basic Definition
The fwrite function writes to an open file. It takes a file handle
and the string to write. An optional length parameter limits the bytes written.
Syntax: fwrite(resource $handle, string $data, int $length = ?): int.
It returns the number of bytes written or false on failure.
Basic fwrite Example
This shows the simplest usage of fwrite to write to a file.
<?php
declare(strict_types=1);
$file = fopen("example.txt", "w");
if ($file) {
$bytes = fwrite($file, "Hello, World!");
fclose($file);
echo "Wrote $bytes bytes to file.";
} else {
echo "Failed to open file.";
}
This creates "example.txt" and writes "Hello, World!" to it. Always check if fopen succeeds and close files when done. The function returns bytes written.
Appending to a File
Use the "a" mode to append data to an existing file without overwriting.
<?php
declare(strict_types=1);
$file = fopen("example.txt", "a");
if ($file) {
$bytes = fwrite($file, "\nAppended line.");
fclose($file);
echo "Appended $bytes bytes to file.";
} else {
echo "Failed to open file.";
}
This appends a new line to "example.txt". The "\n" creates a new line. The "a" mode creates the file if it doesn't exist, like "w" mode.
Writing Binary Data
fwrite can write binary data when the file is opened in binary mode.
<?php
declare(strict_types=1);
$file = fopen("image.png", "wb");
if ($file) {
$binaryData = file_get_contents("source.png");
$bytes = fwrite($file, $binaryData);
fclose($file);
echo "Wrote $bytes bytes of binary data.";
} else {
echo "Failed to open file.";
}
This copies a PNG file by writing binary data. The "b" in mode ensures proper binary handling on Windows. Always use binary mode for non-text files.
Limiting Write Length
The third parameter limits how many bytes are written from the data string.
<?php
declare(strict_types=1);
$file = fopen("limited.txt", "w");
if ($file) {
$data = "This string is too long";
$bytes = fwrite($file, $data, 10);
fclose($file);
echo "Wrote $bytes bytes (limited to 10).";
} else {
echo "Failed to open file.";
}
Only the first 10 bytes ("This strin") are written. The length parameter is useful for writing fixed-size chunks or limiting output size.
Error Handling
Proper error handling is essential when working with file operations.
<?php
declare(strict_types=1);
$file = @fopen("readonly.txt", "w");
if ($file === false) {
echo "Error: Could not open file.";
exit;
}
$bytes = fwrite($file, "Test");
if ($bytes === false) {
echo "Error: Write failed.";
} else {
echo "Successfully wrote $bytes bytes.";
}
fclose($file);
This shows comprehensive error handling. The @ suppresses fopen warnings. Always check return values for false to detect errors in file operations.
Best Practices
- Check Permissions: Ensure files are writable.
- Error Handling: Always verify fwrite return values.
- Resource Cleanup: Close files with fclose when done.
- Binary Safety: Use "b" mode for binary data.
- Locking: Consider flock for concurrent access.
Source
This tutorial covered the PHP fwrite function with practical
examples showing its usage in different scenarios.