PHP fprintf() Function – Complete Guide with Examples

What is fprintf() in PHP?

The fprintf() function in PHP writes formatted data to a file or an output stream, similar to printf(), but instead of displaying output, it writes to a file or resource.

📌 Use cases:
✔ Writing formatted text into files (logs, reports, CSV files).
✔ Sending structured output to streams or sockets.
✔ Formatting log entries with timestamps.

Syntax of fprintf()

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

🔹 Parameters:

$stream → A file handle or resource where data will be written.
$format → A string containing format specifiers (%s, %d, etc.).
$values → The values to be inserted into the format string.

🔹 Return Value:

Returns the number of characters written on success.
Returns false on failure.

Example: Writing Formatted Data to a File

<?php
$file = fopen("output.txt", "w");
if ($file) {
    fprintf($file, "Name: %s, Age: %d\n", "John Doe", 25);
    fclose($file);
    echo "Data written to file.";
} else {
    echo "Failed to open file.";
}
?>

✅ Output (in output.txt):

📌 How it works?

Opens output.txt in write mode (w).
fprintf() writes "John Doe" and 25 into a structured string.
Closes the file using fclose().

Example: Writing Log Entries with Timestamps

<?php
$logFile = fopen("log.txt", "a"); // Open in append mode
if ($logFile) {
    fprintf($logFile, "[%s] ERROR: %s\n", date("Y-m-d H:i:s"), "Database connection failed!");
    fclose($logFile);
    echo "Log entry added.";
} else {
    echo "Failed to open log file.";
}
?>

✅ Output (log.txt):

📌 Why use fprintf() here?

Efficient logging with timestamps.
Avoids manual concatenation of strings.
Uses append mode (a) to prevent overwriting logs.

Example: Creating a CSV File Using fprintf()

<?php
$file = fopen("users.csv", "w");
if ($file) {
    fprintf($file, "ID,Name,Email\n"); // CSV header
    fprintf($file, "%d,%s,%s\n", 1, "Alice", "alice@example.com");
    fprintf($file, "%d,%s,%s\n", 2, "Bob", "bob@example.com");
    fclose($file);
    echo "CSV file created.";
} else {
    echo "Failed to open file.";
}
?>

✅ Output (users.csv):

📌 Why use fprintf() here?

Automates structured CSV file creation.
Ensures consistent formatting.

Writing to the PHP Output Stream (php://stdout)

<?php
fprintf(STDOUT, "Hello, %s!\n", "PHP Developer");
?>

✅ Output (CLI terminal):

📌 Why use STDOUT?

Writes directly to command-line output.
Useful for debugging CLI scripts.

Handling Errors with fprintf()

Example: Writing to a Non-Writable File

<?php
$file = fopen("/system/protected.txt", "w"); // Restricted file path
if ($file) {
    fprintf($file, "This won't work!");
    fclose($file);
} else {
    echo "Error: Cannot open file for writing.";
}
?>

✅ Output (if permission denied):

📌 Best Practices:

Check file permissions before writing.
Handle errors to prevent script failure.

Best Practices for Using fprintf()

✅ 1. Always Close Files
Use fclose($file); to release resources after writing.

✅ 2. Use a (append) Mode for Logs
Prevents overwriting logs when writing new entries.

✅ 3. Check File Permissions
Ensure your script has write access before attempting to write.

✅ 4. Use STDOUT for CLI Scripts
Great for debugging and command-line output.

✅ 5. Format Strings Properly
Use:

%s for strings
%d for integers
%f for floating-point numbers

Alternative Functions to fprintf()

Function	Description
`printf()`	Formats and outputs a string to the screen.
`sprintf()`	Formats a string and returns it.
`file_put_contents()`	Writes a string to a file without formatting.
`fwrite()`	Writes raw data to a file without formatting.

📌 Use fprintf() when formatting is required before writing to a file or stream.

When to Use fprintf()

📌 Use fprintf() when:
✔ Writing formatted output to files or streams.
✔ Storing structured logs, reports, or CSV files.
✔ Using PHP CLI scripts with STDOUT.

⚠️ Avoid if:
❌ You only need to print to the screen → Use printf().
❌ You don’t need formatting → Use fwrite().