PHP header_register_callback() Function: Usage and Examples

Use header_register_callback() when you need to registers a callback invoked before PHP sends response headers in a PHP application. The examples below show the actual operation and its limitations.

What Is header_register_callback() in PHP?

Registers a callback invoked before PHP sends response headers. The exact signature supported by your installation depends on its PHP version and enabled extensions.

Its syntax is:

header_register_callback(callable $callback): bool

Parameters and return value: The parameter types, optional arguments, references, and return type are shown in the signature above. Optional parameters are marked with an ellipsis in this reference signature.

Example 1: Basic Usage

<?php

if (PHP_SAPI === "cli") { echo "Requires HTTP runtime"; } else { header_register_callback(function () { header("X-App-Stage: ready"); }); }

Result: Run the example in an environment that supports the function. Any values depending on operating-system state, installed extensions, process configuration, or network access must be checked on the target server.

Example 2: Different Input or Behavior

<?php

if (PHP_SAPI !== "cli" && !headers_sent()) { header_register_callback(fn() => header("X-Demo: active")); } else { echo "Browser request required"; }

Result: Run the example in an environment that supports the function. Any values depending on operating-system state, installed extensions, process configuration, or network access must be checked on the target server.

Practical Example: Applying the Function

<?php

$callback = function () { header("Cache-Control: private, no-store"); }; if (PHP_SAPI !== "cli" && !headers_sent()) { header_register_callback($callback); } else { echo "HTTP headers unavailable here"; }

Result: Run the example in an environment that supports the function. Any values depending on operating-system state, installed extensions, process configuration, or network access must be checked on the target server.

Important Behavior and Edge Cases

Callback timing is just before headers are sent; do not perform slow or recursive output operations there.

Comparison With Related Functions

Common Beginner Mistakes

Key Takeaways

• Registers a callback invoked before PHP sends response headers.
• Callback timing is just before headers are sent; do not perform slow or recursive output operations there.
• header() sets a header immediately; header_register_callback() schedules work before headers are sent.

Use header_register_callback() for the specific operation demonstrated above. Check its prerequisites and handle failure conditions before using it with production data.