How to ignore invalid SSL certificate errors in Guzzle 5
You should use
$this->client = new GuzzleClient(['defaults' => [ 'verify' => false]]);i.e. a Boolean false, not the string 'false'
The documentation is here:https://docs.guzzlephp.org/en/5.3/clients.html#verify
Note: a few people have given other answers that apply to Guzzle 6+. They're good answers if you are using those versions (but the original question was explicitly about Guzzle 5).