src/core/Request.php
public function getHeader(string $name): ?string
    {
        $name = strtolower($name);
        foreach ($this->getAllHeaders() as $key => $value) {
            if (strtolower($key) === $name) {
                return $value;
            }
        }
        return null;
    }
public function getHeader(string $name): ?string
    {
        $name = strtolower($name);
        foreach ($this->getAllHeaders() as $key => $value) {
            if (strtolower($key) === $name) {
                return $value;
            }
        }
        return null;
    }
It returns the header by passed name. The search is case-insensitive.
@param string $name
@return string|null