PHP fprintf() Function

What You’ll Learn

The fprintf() function writes a formatted string directly to a stream—typically a file opened with fopen(). It shares format specifiers with sprintf() and printf(), but targets a file handle instead of returning or echoing text.

Definition and Usage

In PHP, fprintf() combines string formatting with stream I/O. You pass a stream resource (from fopen()), a format string with placeholders, and the values to insert. PHP writes the formatted result to the stream and returns how many bytes were written.

📝 Syntax

The fprintf() function requires a stream, a format string, and optional values:

int fprintf(resource $stream, string $format, mixed ...$values)

Parameters

• $stream — a file pointer resource from fopen() (file, php://memory, etc.). Must be opened in a write-capable mode.
• $format — format string with conversion specifiers like %s (string), %d (integer), %.2f (float with 2 decimals).
• ...$values — values inserted into the format placeholders in order.

Return Value

Returns the number of bytes written to the stream. Since PHP 8.0, invalid arguments throw ValueError or ArgumentCountError instead of returning false.

Examples Gallery

Run these examples in PHP 7+. Each demonstrates a common pattern beginners encounter with fprintf().

<?php
$stream = fopen('php://memory', 'rw+');

$bytes = fprintf($stream, "Hello, %s!", "PHP");

rewind($stream);
$output = stream_get_contents($stream);
fclose($stream);

echo "Written: $output\n";
echo "Bytes:   $bytes\n";
?>

Written: Hello, PHP!
Bytes:   11

<?php
$stream = fopen('php://memory', 'rw+');

$year  = 2005;
$month = 5;
$day   = 6;

fprintf($stream, "%04d-%02d-%02d", $year, $month, $day);

rewind($stream);
echo stream_get_contents($stream) . "\n";

fclose($stream);
?>

2005-05-06

<?php
$stream = fopen('php://memory', 'rw+');

$price1 = 68.75;
$price2 = 54.35;
$total  = $price1 + $price2;

$bytes = fprintf($stream, '%.2f', $total);

rewind($stream);
$written = stream_get_contents($stream);
fclose($stream);

echo "Total:   $total\n";
echo "Written: $written\n";
echo "Bytes:   $bytes\n";
?>

Total:   123.1
Written: 123.10
Bytes:   6

<?php
$stream = fopen('php://memory', 'rw+');

$entries = [
    ["09:15:00", "INFO",  "User login",  "alice"],
    ["09:15:01", "WARN",  "Slow query",  "orders"],
    ["09:15:02", "ERROR", "DB timeout",  "mysql"],
];

foreach ($entries as [$time, $level, $msg, $ctx]) {
    fprintf($stream, "[%s] %-5s | %s (%s)\n", $time, $level, $msg, $ctx);
}

rewind($stream);
echo stream_get_contents($stream);
fclose($stream);
?>

[09:15:00] INFO  | User login (alice)
[09:15:01] WARN  | Slow query (orders)
[09:15:02] ERROR | DB timeout (mysql)

<?php
$name = "CodeToFun";

// sprintf — returns a string
$text = sprintf("Welcome to %s!", $name);
echo "sprintf:  $text\n";

// fprintf — writes to a stream, returns byte count
$stream = fopen('php://memory', 'rw+');
$bytes  = fprintf($stream, "Welcome to %s!", $name);
rewind($stream);
echo "fprintf:  " . stream_get_contents($stream) . " ($bytes bytes)\n";
fclose($stream);

// printf — echoes directly (commented to keep output tidy)
// printf("Welcome to %s!\n", $name);
echo "printf:   echoes directly to output\n";
?>

sprintf:  Welcome to CodeToFun!
fprintf:  Welcome to CodeToFun! (21 bytes)
printf:   echoes directly to output

🚀 Common Use Cases

• Log files — append timestamped, formatted entries with fopen('app.log', 'a').
• CSV and data exports — write fixed-width or delimited rows efficiently.
• Report generation — produce formatted text files (dates, currency, IDs).
• Incremental buffers — build large strings in memory via php://memory.
• CLI scripts — write formatted output to STDERR or custom streams.

📝 Notes

• The first argument must be a stream resource, not a plain string (fix from older tutorials).
• Always fclose() streams when finished to free resources.
• Since PHP 8.0, missing format arguments throw ArgumentCountError.
• Use 'a' mode to append logs; 'w' overwrites existing files.
• For simple string building without files, sprintf() is often clearer.

💡 Best Practices