Hello Dani,
Thanks for the answer (and sorry for my late answer to yours).
In my plugin I used to use Guzzle to easily call functions which were in another library I added thanks to a local composer file. I have no problem in my browser and the plugin works well but when I tried to test it on the moodle mobile app I encounted the error (probably because of what you say about the app who doesn't include any external lib installed in Moodle).
Thanks for the answer (and sorry for my late answer to yours).
In my plugin I used to use Guzzle to easily call functions which were in another library I added thanks to a local composer file. I have no problem in my browser and the plugin works well but when I tried to test it on the moodle mobile app I encounted the error (probably because of what you say about the app who doesn't include any external lib installed in Moodle).
For that I edited my plugin to have two different behiavours in function of if we call it classicly (web, browser) or if we call it via the mobile app. In this case I do not use guzzle and my other library and I do some curl directly on the API. It is a little more complicated because I cannot use existant functions of my library but I think I managed to do what I needed.
For information I test the source by using this:
if (WS_SERVER) { // Curl calls. } else { // Guzzle calls. }