Use headers_list() when you need to returns the headers PHP has scheduled for the HTTP response in a PHP application. The examples below show the actual operation and its limitations.
What Is headers_list() in PHP?
Returns the headers PHP has scheduled for the HTTP response. The exact signature supported by your installation depends on its PHP version and enabled extensions.
Its syntax is:
headers_list(): arrayParameters 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
header("X-Demo: sample"); echo is_array(headers_list()) ? "List available" : "Unavailable";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
echo count(headers_list()) >= 0 ? "Headers inspected" : "Error";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
if (!headers_sent()) header("X-Trace: test"); foreach (headers_list() as $headerValue) { if (str_starts_with($headerValue,"X-Trace:")) echo $headerValue; }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
Headers visible in CLI are not representative of a real web-server response.
Comparison With Related Functions
| Approach | Difference |
|---|---|
headers_list() | Returns the headers PHP has scheduled for the HTTP response. |
| Related operation | headers_sent() reports whether output headers have been sent; headers_list() lists pending headers. |
Common Beginner Mistakes
| Mistake | Correct Approach |
|---|---|
| Ignoring the function-specific limitation | Headers visible in CLI are not representative of a real web-server response. |
| Confusing this function with a related operation | headers_sent() reports whether output headers have been sent; headers_list() lists pending headers. |
Key Takeaways
- Returns the headers PHP has scheduled for the HTTP response.
- Headers visible in CLI are not representative of a real web-server response.
- headers_sent() reports whether output headers have been sent; headers_list() lists pending headers.
Use headers_list() for the specific operation demonstrated above. Check its prerequisites and handle failure conditions before using it with production data.