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In PHP file handling functions, fpassthru() is an understated yet highly practical function. Its primary role is to read from an open file pointer until the end of the file and output the content directly. When combined with PHP’s output buffering mechanism, the behavior of fpassthru() can be influenced, affecting output efficiency and logical control. This article introduces the basic usage of fpassthru() and explores its relationship with output buffering, along with strategies for leveraging it to optimize PHP output.
The function prototype of fpassthru() is as follows:
<span><span><span class="hljs-keyword">int</span></span><span> </span><span><span class="hljs-title function_ invoke__">fpassthru</span></span><span> ( resource </span><span><span class="hljs-variable">$handle</span></span><span> )
</span></span>It accepts a valid file resource pointer (usually returned by fopen()), reads from the current position to the end of the file, and sends the content directly to the output buffer or client. It is typically used to send file content directly to the browser, such as for downloading files or displaying logs.
Example:
<span><span><span class="hljs-variable">$fp</span></span><span> = </span><span><span class="hljs-title function_ invoke__">fopen</span></span><span>(</span><span><span class="hljs-string">"example.txt"</span></span><span>, </span><span><span class="hljs-string">"r"</span></span><span>);
</span><span><span class="hljs-keyword">if</span></span><span> (</span><span><span class="hljs-variable">$fp</span></span><span>) {
</span><span><span class="hljs-comment">// Optional: set the correct header</span></span><span>
</span><span><span class="hljs-title function_ invoke__">header</span></span><span>(</span><span><span class="hljs-string">"Content-Type: text/plain"</span></span><span>);
</span><span><span class="hljs-title function_ invoke__">fpassthru</span></span><span>(</span><span><span class="hljs-variable">$fp</span></span><span>);
</span><span><span class="hljs-title function_ invoke__">fclose</span></span><span>(</span><span><span class="hljs-variable">$fp</span></span><span>);
}
</span></span>PHP’s output buffering mechanism allows developers to control when output is sent to the client. Without buffering, PHP sends content to the browser immediately each time echo, print, or similar functions are executed. However, when output buffering is started using ob_start(), all output is temporarily stored in the buffer until ob_end_flush(), ob_flush(), or the script ends.
An important function of output buffering is that it allows developers to modify content, add headers, or even cancel part of the output before sending it, offering more flexible control over content presentation.
fpassthru() does not bypass the output buffer—it still writes data to the buffer rather than sending it directly to the client. This means that if output buffering is enabled, content from fpassthru() will also be buffered.
Consequently, when using fpassthru(), if output buffering is enabled (for example, with ob_start()), you must flush or clear the buffer at the appropriate time; otherwise, the user may not see the output immediately, or a “headers already sent” error may occur before output.
Example:
<span><span><span class="hljs-title function_ invoke__">ob_start</span></span><span>();
<p></span>$fp = fopen("largefile.zip", "rb");<br>
if ($fp) {<br>
header("Content-Type: application/zip");<br>
header("Content-Disposition: attachment; filename="download.zip"");<br>
fpassthru($fp);<br>
fclose($fp);<br>
}</p>
<p>ob_end_flush(); // Content is sent to the client only at this point<br>
</span>If ob_end_flush() is not called, content may remain in the buffer for a long time, which can affect user experience, especially when outputting large files.
When using fpassthru() efficiently and securely, consider the following points:
If you need immediate output (such as downloading large files), it is recommended to disable output buffering or flush it manually.
You can use ob_end_clean() to clear the buffer to avoid conflicts.
<span><span><span class="hljs-comment">// Avoid extra output interfering with file download</span></span><span>
</span><span><span class="hljs-keyword">while</span></span><span> (</span><span><span class="hljs-title function_ invoke__">ob_get_level</span></span><span>()) {
</span><span><span class="hljs-title function_ invoke__">ob_end_clean</span></span><span>();
}
</span></span>When using fpassthru() to output files, always set headers such as Content-Type and Content-Disposition beforehand. Once output begins, setting headers will fail.
Before calling fpassthru(), ensure no other HTML, spaces, BOM, or error messages are output. These can pollute the output stream and break scenarios like file downloads.
Although fpassthru() is fundamentally a simple output function, its efficiency in handling large file transfers and integration with output buffering makes it a valuable tool in PHP file handling. Understanding and properly using output buffering allows developers to build more controlled and high-performance output logic, especially in scenarios that require managing client response content.
By combining practical needs with a thoughtful arrangement of buffering and output functions, PHP developers can control application output more flexibly, achieving a more professional content presentation.
