Use headers_sent() when you need to checks whether HTTP headers have already been transmitted in a PHP application. The examples below show the actual operation and its limitations.
What Is headers_sent() in PHP?
Checks whether HTTP headers have already been transmitted. The exact signature supported by your installation depends on its PHP version and enabled extensions.
Its syntax is:
headers_sent(&$filename = ..., &$line = ...): boolParameters 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
var_dump(headers_sent());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 headers_sent() ? "Headers sent" : "Headers available";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($sourceFile,$sourceLine)) { header("X-Demo: ready"); echo "Header set"; } else { echo "Headers sent at $sourceFile:$sourceLine"; }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
Once headers are transmitted you cannot safely add or replace them using header().
Comparison With Related Functions
| Approach | Difference |
|---|---|
headers_sent() | Checks whether HTTP headers have already been transmitted. |
| Related operation | headers_list() lists scheduled headers; headers_sent() checks the sent state. |
Common Beginner Mistakes
| Mistake | Correct Approach |
|---|---|
| Ignoring the function-specific limitation | Once headers are transmitted you cannot safely add or replace them using header(). |
| Confusing this function with a related operation | headers_list() lists scheduled headers; headers_sent() checks the sent state. |
Key Takeaways
- Checks whether HTTP headers have already been transmitted.
- Once headers are transmitted you cannot safely add or replace them using header().
- headers_list() lists scheduled headers; headers_sent() checks the sent state.
Use headers_sent() for the specific operation demonstrated above. Check its prerequisites and handle failure conditions before using it with production data.