fprint()

The PHP fprintf() function writes a formatted string to an open file (stream). Think of it as printf() — which prints to the browser/output — but with the destination redirected to a file handle. It is the natural choice when you want clean, aligned, predictable text in a log file or a generated report, instead of building strings by hand with concatenation.

This page covers the syntax, the format specifiers you'll use most, two complete runnable-style examples, and the closely related functions you can reach for instead.

Syntax

int fprintf ( resource $handle , string $format [, mixed ...$args ] )

The function returns the number of bytes written, or false on failure. Note that the return value is the length of the written string, not the number of arguments — useful when you need to track how much data you've logged.

Format specifiers

The power of fprintf() is in the $format string. Each % introduces a placeholder:

The same specifiers work in printf() and sprintf(), so once you learn them here they carry over everywhere.

Writing a report

Here is a complete example that writes a small report to a file.

<?php
$file = fopen("report.txt", "w");
if ($file === false) {
    die("Failed to open file");
}
$name = "John";
$age = 30;
$income = 50000;
// \$ prints a literal dollar sign; %.2f rounds the float to 2 decimals
$bytes = fprintf($file, "Name: %s\nAge: %d\nIncome: \$%.2f", $name, $age, $income / 12);
fclose($file);

echo "$bytes bytes written"; // 35 bytes written
?>

We open report.txt for writing ("w") and check that fopen() did not fail. fprintf() then substitutes each variable into the format string: %s for the name, %d for the age, and \$%.2f for the monthly income (a literal $ followed by a float rounded to two decimals). Always call fclose() when you're done so the buffer is flushed and the handle is released.

The resulting report.txt contains:

Name: John
Age: 30
Income: $4166.67

50000 / 12 is 4166.6667, which %.2f rounds to 4166.67.

Appending to a log file

A very common use is appending one formatted line to a log. Open the file in append mode ("a") so each call adds a new line instead of overwriting the previous content:

<?php
$file = fopen("error.log", "a");
if ($file === false) {
    die("Failed to open file");
}
$error_code = 404;
$date = date("Y-m-d H:i:s");
$ip_address = $_SERVER['REMOTE_ADDR'];
fprintf($file, "[%s] Error %d from IP address %s\n", $date, $error_code, $ip_address);
fclose($file);
?>

Because the file is opened with "a", each run appends one new line; nothing already in error.log is lost. The \n at the end of the format string puts each entry on its own line. A typical appended line looks like:

[2023-03-15 15:30:00] Error 404 from IP address 192.168.0.1

fprintf() vs. related functions

Choosing the right tool keeps your code clear:

• printf() — same formatting, but writes to standard output (the browser/CLI) instead of a file.
• sprintf() — returns the formatted string instead of writing it anywhere, so you can store or reuse it.
• fwrite() — writes raw bytes to a file with no formatting; use it when you don't need % placeholders.
• vfprintf() — like fprintf(), but takes the arguments as a single array instead of a variadic list.

Common gotchas

• Mismatched specifiers and arguments. %d on a non-numeric string yields 0; passing fewer arguments than placeholders triggers a warning.
• Forgetting to escape %. To print a literal percent sign, use %%, not %.
• Forgetting fclose(). Buffered data may not be flushed to disk until the handle is closed (or the script ends).
• Open mode matters. "w" truncates the file; "a" appends. Pick deliberately.

Practice