wp-includes/rest-api/class-wp-rest-server.php:1347Returns if an error occurred during most recent JSON encode/decode.
false|string protected function get_json_last_error() { $last_error_code = json_last_error(); if ( JSON_ERROR_NONE === $last_error_code || empty( $last_error_code ) ) { return false; } return json_last_error_msg(); }Introduced in 4.4.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/rest-api/class-wp-rest-server.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.