write the watches and edits guide page

[carlos-granados]

Replaces the watch expressions placeholder with a page covering both watch expressions and modifying values, since the two share a mechanism.

Renamed watch-expressions.md to watches-and-edits.md. Only the sidebar referenced the old id and the page had never held real content, so no redirect was added — /user-guide/watch-expressions/ will 404.

Verified against handler_dbgp.c and debugger.c:

• a watch is zend_eval_string in the paused process, so side effects are real
• breakpoints_allowed is set to 0 during evaluation, so breakpoints inside a watch never fire
• error_reporting is set to 0 for the evaluation, so warnings and notices are swallowed; a thrown exception is returned as its message
• eval takes no depth argument and uses the current execute data, so watches evaluate in the frame where execution stopped rather than the frame selected in the call stack. property_set does swap frames, so edits honour the selection and watches do not — the page calls this out.
• editing a value is an assignment run through the same eval, with an optional bool / int / float / string cast, reporting only success or failure

[github-actions]

PR Preview Action v1.8.1
Preview removed because the pull request was closed.
2026-09-06 06:46 UTC