[wp-trac] [WordPress Trac] #64108: Triggered errors are not displayed when applying template enhancement output buffer filters

[WordPress Trac]

#64108: Triggered errors are not displayed when applying template enhancement
output buffer filters
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 Reporter:  westonruter   |      Owner:  westonruter
     Type:  defect (bug)  |     Status:  assigned
 Priority:  normal        |  Milestone:  6.9
Component:  General       |    Version:  trunk
 Severity:  normal        |   Keywords:
  Focuses:                |
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 This is follow up to #43258.

 Attempting to `echo` during a user output buffer callback does not add
 anything to the output, and this is now deprecated in PHP 8.5 as
 [https://core.trac.wordpress.org/ticket/43258#comment:73 discovered] by
 @swissspidy (see initial fix in [60945]), and was
 [https://core.trac.wordpress.org/ticket/43258#comment:74 noted] by
 @jorbin:

 > More info on the deprecation: https://github.com/php/php-
 src/commit/07f1cfd9b01ff0f3720c1a5580b9e263eec5fce1 and
 https://wiki.php.net/rfc/deprecations_php_8_4#:~:text=has%20been%20closed.-,Deprecate%20producing%20output%20in%20a%20user%20output%20handler,buffering%20function%20in%20an%20output%20handler%20will%20emit%20a%20Fatal%20Error.,-Deprecate%20producing%20output

 Of note:

 > '''Deprecate producing output in a user output handler'''
 >
 > Because an output handler should just be manipulating the incoming
 buffer, any output that it produces is discarded. As such, issues within
 an output handler can go unnoticed and be hard to debug.
 >
 > Therefore, we propose deprecating producing any output in an output
 handler, in the same way as attempting to use an output buffering function
 in an output handler will emit a Fatal Error.

 Currently when `WP_DEBUG_DISPLAY` is enabled (and thus `display_errors` is
 on), calls to `_doing_it_wrong()` and `wp_trigger_error()` in callbacks
 for the `wp_template_enhancement_output_buffer` filter are problematic. In
 reality, this has been a problem already for plugins that open their own
 output buffers and either trigger errors in their output buffer callback
 or provide extensibility for other plugins which can cause errors.

 We should better handle errors triggered during a user output handler. We
 capture with an error handler and then when `display_errors` is on we can
 append the errors as HTML to the response.

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Ticket URL: <https://core.trac.wordpress.org/ticket/64108>
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