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).