PHP fpassthru Function

last modified April 3, 2025

The PHP fpassthru function outputs all remaining data from a file pointer. It’s efficient for streaming large files directly to output.

Basic Definition

The fpassthru function reads from the current position in a file until EOF and writes the data to the output buffer. It returns bytes read.

Syntax: fpassthru(resource $stream): int. The function is useful for binary files and requires an already opened file pointer.

Basic fpassthru Example

This shows the simplest usage of fpassthru to output a file.

basic_fpassthru.php

<?php

declare(strict_types=1);

$file = fopen(’example.txt’, ‘rb’); if ($file) { fpassthru($file); fclose($file); }

This reads and outputs the entire contents of example.txt. The file is opened in binary mode for reliable reading across platforms.

Outputting Image Files

fpassthru is commonly used to output binary files like images.

image_output.php

<?php

declare(strict_types=1);

$imagePath = ‘photo.jpg’; $file = fopen($imagePath, ‘rb’);

if ($file) { header(‘Content-Type: image/jpeg’); fpassthru($file); fclose($file); exit; }

This example streams a JPEG image directly to the browser. Proper headers must be set before calling fpassthru for binary data.

Partial File Output

You can combine fseek with fpassthru for partial output.

partial_output.php

<?php

declare(strict_types=1);

$file = fopen(’largefile.bin’, ‘rb’); if ($file) { fseek($file, 1024); // Skip first 1KB $bytes = fpassthru($file); echo “Output $bytes bytes”; fclose($file); }

This skips the first 1KB of the file before outputting the rest. The function returns the number of bytes output, which can be useful for logging.

PDF File Download

fpassthru can force file downloads with proper headers.

pdf_download.php

<?php

declare(strict_types=1);

$pdfFile = ‘document.pdf’; $file = fopen($pdfFile, ‘rb’);

if ($file) { header(‘Content-Type: application/pdf’); header(‘Content-Disposition: attachment; filename=“downloaded.pdf”’); fpassthru($file); fclose($file); exit; }

This forces a PDF download in the browser. The Content-Disposition header tells the browser to save rather than display the file.

Error Handling

Proper error handling is important when working with file operations.

error_handling.php

<?php

declare(strict_types=1);

$filePath = ‘data.txt’;

if (!file_exists($filePath)) { die(“File not found”); }

$file = fopen($filePath, ‘rb’); if (!$file) { die(“Failed to open file”); }

if (fpassthru($file) === false) { die(“Error reading file”); }

fclose($file);

This example includes basic error checking for file existence, opening, and reading. Always verify operations when working with files.

Best Practices

• Close Files: Always close files after using fpassthru.

• Binary Mode: Use ‘rb’ mode for reliable binary reading.

• Headers First: Set output headers before fpassthru.

• Memory Efficiency: Use for large files to avoid memory issues.

• Security: Validate file paths to prevent directory traversal.

Source

PHP fpassthru Documentation

This tutorial covered the PHP fpassthru function with practical examples showing its usage for efficient file output.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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