If you want cURL to successfully write cookies to a file specified with CURLOPT_COOKIEJAR, ensure that cURL has the necessary permissions to modify the file if it already exists.
I spent nearly a day trying to understand why cURL wasn't saving cookies to an existing file, even though I could easily modify the exact same file using file_put_contents(). Moreover, cURL itself could create the same file and save cookies, but only if it didn't previously exist.
Ultimately, the issue was related to file ownership. I was working within WSL2, inside a symlinked Windows directory. The [automount]"metadata" in wsl.conf was not set, causing every file created from PHP to have the default owner, which differed from the user running PHP.
Once I configured wsl.conf and then changed the ownership of the entire directory to match the user running PHP, cookies were successfully written to any file without any issues.
