Sandbox cURL (PHP 5)

[SammyK]

Description

This is a follow up PR to #814. This sandboxes the cURL integration on PHP 5.6 (PHP 5.4 is still limited to the legacy API so it does not apply.)

As in #814, this PR also provides custom internal function handlers for curl_exec, curl_copy_handle, curl_close, curl_setopt, and curl_setopt_array in order to inject distributed tracing headers. This PR differs slightly from #814 by injecting the distributed tracing headers with a single userland call to DDTrace\Bridge\curl_inject_distributed_headers() right before the curl_exec() call. This approach can be used for PHP 7 to reduce the dependency on the userland API.

Readiness checklist

• (only for Members) Changelog has been added to the appropriate release draft. Create one if necessary.
• Tests added for this feature/bug.

Reviewer checklist

• Appropriate labels assigned.
• Milestone is set.
• Changelog has been added to the appropriate release draft. For community contributors the reviewer is in charge of this task.

[morrisonlevi]

Testing if static analysis is required for merge.

[morrisonlevi]

Looks good, Sammy. Thanks for wading through PHP 5!