Upload of pre-existing files

PHP/PHP PSR-7.md

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+# PSR-7
+
+## [PSR-7](https://www.php-fig.org/psr/psr-7/)
+
+Standard of the PHP Framework Interop Group that defines common interafces for handling HTTP messages.
+
+- `Psr\Http\Message\MessageInterface`
+- `Psr\Http\Message\RequestInterface`
+- `Psr\Http\Message\ResponseInterface`
+- `Psr\Http\Message\ServerRequestInterface`
+- `Psr\Http\Message\StreamInterface`
+- `Psr\Http\Message\UploadedFileInterface`
+- `Psr\Http\Message\UriInterface`
+
+Example:
+
+```php
+// empty array if not found
+$header = $request->getHeader('Accept');
+
+// empty string if not found
+$header = $request->getHeaderLine('Accept');
+
+// check the presence of a header
+if (! $request->hasHeader('Accept')) {}
+
+// returns the parameters in a query string
+$query = $request->getQueryParams();
+```
+
+### Immutability
+
+PSR-7 requests are *immutable* objects; a change in the data will return a new instance of the object.
+The stream objects of PSR-7 are *not immutable*.