PHP Manual: Binary-safe file write

Description

intfalse fwrite(resource $stream, string $data, intnull $length = null)

fwrite writes the contents of data to the file stream pointed to by stream.

Parameters

stream: A file system pointer resource that is typically created using fopen.
data: The string that is to be written.
length: If length is an int, writing will stop after length bytes have been written or the end of data is reached, whichever comes first.

Return Values

fwrite returns the number of bytes written, or false on failure.

Errors/Exceptions

fwrite raises E_WARNING on failure.

Changelog

Version	Description
8.0.0	`length` is nullable now.

Examples

Example #1 A simple fwrite example

<?php
$filename = 'test.txt';
$somecontent = "Add this to the file\n";

// Let's make sure the file exists and is writable first.
if (is_writable($filename)) {

    // In our example we're opening $filename in append mode.
    // The file pointer is at the bottom of the file hence
    // that's where $somecontent will go when we fwrite() it.
    if (!$fp = fopen($filename, 'a')) {
         echo "Cannot open file ($filename)";
         exit;
    }

    // Write $somecontent to our opened file.
    if (fwrite($fp, $somecontent) === FALSE) {
        echo "Cannot write to file ($filename)";
        exit;
    }

    echo "Success, wrote ($somecontent) to file ($filename)";

    fclose($fp);

} else {
    echo "The file $filename is not writable";
}
?>

Notes

Note:

Writing to a network stream may end before the whole string is written. Return value of fwrite may be checked:

<?php
function fwrite_stream($fp, $string) {
    for ($written = 0; $written < strlen($string); $written += $fwrite) {
        $fwrite = fwrite($fp, substr($string, $written));
        if ($fwrite === false) {
            return $written;
        }
    }
    return $written;
}
?>

Note:
On systems which differentiate between binary and text files (i.e. Windows) the file must be opened with 'b' included in fopen mode parameter.

Note:
If stream was fopened in append mode, fwrites are atomic (unless the size of data exceeds the filesystem's block size, on some platforms, and as long as the file is on a local filesystem). That is, there is no need to flock a resource before calling fwrite; all of the data will be written without interruption.

Note:
If writing twice to the file pointer, then the data will be appended to the end of the file content:
<?php
$fp = fopen('data.txt', 'w');
fwrite($fp, '1');
fwrite($fp, '23');
fclose($fp);

// the content of 'data.txt' is now 123 and not 23!
?>

fwrite