src/AbstractTransport.php
public function setHeader($keyName, $value)
    {
        $keyName = $this->normalizeHeaderName($keyName);
        if (!$keyName) {
            return $this;
        }

        if (!isset($this->configs['headers'])) {
            $this->configs['headers'] = [];
        }

        $this->configs['headers'][$keyName] = [$value];
        if (!$this->inProcessingLoop) {
            return $this
                ->buildConfigClient()
                ->withRequest($this->request->withHeader($keyName, $value));
        }

        return $this
            ->withRequest($this->request->withHeader($keyName, $value));
    }
public function setHeader($keyName, $value)
    {
        $keyName = $this->normalizeHeaderName($keyName);
        if (!$keyName) {
            return $this;
        }

        if (!isset($this->configs['headers'])) {
            $this->configs['headers'] = [];
        }

        $this->configs['headers'][$keyName] = [$value];
        if (!$this->inProcessingLoop) {
            return $this
                ->buildConfigClient()
                ->withRequest($this->request->withHeader($keyName, $value));
        }

        return $this
            ->withRequest($this->request->withHeader($keyName, $value));
    }
Set Header

@param string $keyName Header Name key, The key will be convert into
First Character after `-`(dash) into uppercase
And space will be replace as `dash`
@param string $value
@return static