== How to fix it in Core PHP

=== Code examples

The following code is vulnerable to cross-site scripting because it returns an HTML response that contains user input.

User input embedded in HTML code should be HTML-encoded to prevent the injection of additional code.
PHP provides the built-in function `htmlspecialchars` to do this.

==== Noncompliant code example

[source,php,diff-id=1,diff-type=noncompliant]
----
echo '<h1>' . $input . '</h1>';
----

==== Compliant solution

[source,php,diff-id=1,diff-type=compliant]
----
echo '<h1>' . htmlspecialchars($input) . '</h1>';
----

If you do not intend to send HTML code to clients, the vulnerability can be fixed by specifying the type of data returned in the response with the content-type header.

For example, setting the content-type to `text/plain` using the built-in `header` function allows to safely reflect user input since browsers will not try to parse and execute the response.

==== Noncompliant code example

[source,php,diff-id=2,diff-type=noncompliant]
----
echo $input;
----

==== Compliant solution

[source,php,diff-id=2,diff-type=compliant]
----
header('Content-Type: text/plain');
echo $input;
----

=== How does this work?

include::../../common/fix/data_encoding.adoc[]

=== Pitfalls

include::../../common/pitfalls/content-types.adoc[]

==== Single quoted variables in attributes

By default, `htmlspecialchars` does not encode single quotes, so if `++$input++` is untrusted, JavaScript code can be injected.

Make sure to set the option `ENT_QUOTES` to encode single quotes.

===== Noncompliant code example

[source,php,diff-id=3,diff-type=noncompliant]
----
echo "<img src='" . htmlspecialchars($input) . "'>";
----

===== Compliant solution

[source,php,diff-id=3,diff-type=compliant]
----
echo "<img src='" . htmlspecialchars($input, ENT_QUOTES) . "'>";
----

==== Headers and output

If the HTTP body is sent before `header` is called, no headers will be sent to the client.

To fix this issue, send the headers before any output.

===== Noncompliant code example

[source,php,diff-id=4,diff-type=noncompliant]
----
echo 'No more headers at this point';
header('Content-Type: text/plain');
echo $input;
----

===== Compliant solution

[source,php,diff-id=4,diff-type=compliant]
----
header('Content-Type: text/plain');
echo $input;
----

=== Going the extra mile

include::../../common/extra-mile/csp.adoc[]