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<?php echo preg_replace_callback('/([a-z])([A-Z])/', '$1_$2', 'eOaUi'); echo preg_replace_callback('/([a-z])([A-Z])/', function (array $matches) { var_dump($matches); }, 'eOaUi');
Output for 8.0.0 - 8.0.30, 8.1.0 - 8.1.27, 8.2.0 - 8.2.18, 8.3.0 - 8.3.6
Fatal error: Uncaught TypeError: preg_replace_callback(): Argument #2 ($callback) must be a valid callback, function "$1_$2" not found or invalid function name in /in/RAVFk:3 Stack trace: #0 /in/RAVFk(3): preg_replace_callback('/([a-z])([A-Z])...', '$1_$2', 'eOaUi') #1 {main} thrown in /in/RAVFk on line 3
Process exited with code 255.
Output for 5.3.0 - 5.3.29, 5.4.0 - 5.4.45, 5.5.0 - 5.5.38, 5.6.0 - 5.6.28, 7.0.0 - 7.0.20, 7.1.0 - 7.1.33, 7.2.0 - 7.2.33, 7.3.0 - 7.3.33, 7.4.0 - 7.4.33
Warning: preg_replace_callback(): Requires argument 2, '$1_$2', to be a valid callback in /in/RAVFk on line 3 eOaUiarray(3) { [0]=> string(2) "eO" [1]=> string(1) "e" [2]=> string(1) "O" } array(3) { [0]=> string(2) "aU" [1]=> string(1) "a" [2]=> string(1) "U" } i
Output for 4.4.2 - 4.4.9, 5.1.0 - 5.1.6, 5.2.0 - 5.2.17
Parse error: syntax error, unexpected T_FUNCTION in /in/RAVFk on line 5
Process exited with code 255.
Output for 4.3.0 - 4.3.1, 4.3.5 - 4.3.11, 4.4.0 - 4.4.1, 5.0.0 - 5.0.5
Parse error: parse error, unexpected T_FUNCTION in /in/RAVFk on line 5
Process exited with code 255.
Output for 4.3.2 - 4.3.4
Parse error: parse error in /in/RAVFk on line 5
Process exited with code 255.

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