Custom Error Handlers (set_error_handler)

TL;DR

Explanation

PHP triggers errors via a global error handler. set_error_handler(callable $handler) replaces it for user-level errors (E_WARNING, E_NOTICE, E_USER_ERROR etc.) but cannot catch E_ERROR, E_PARSE, or E_CORE_ERROR — those still kill the script unless register_shutdown_function() is used. The callback receives ($errno, $errstr, $errfile, $errline). Always restore the previous handler with restore_error_handler() in tests. set_exception_handler() is the equivalent for uncaught exceptions.

Common Misconception

Why It Matters

Common Mistakes

• Forgetting to return true from the handler — PHP then executes the internal handler too.
• Not restoring the previous handler — breaks test isolation.
• Using set_error_handler() alone without register_shutdown_function() — misses fatal errors.

Code Examples

set_error_handler(function($errno, $errstr) {
    // Silently ignores — errors disappear
});

set_error_handler(function(int $errno, string $errstr, string $errfile, int $errline): bool {
    throw new \ErrorException($errstr, 0, $errno, $errfile, $errline);
});

register_shutdown_function(function() {
    $error = error_get_last();
    if ($error && in_array($error['type'], [E_ERROR, E_PARSE, E_CORE_ERROR])) {
        // Log fatal error before script dies
        error_log("Fatal: {$error['message']} in {$error['file']}:{$error['line']}");
    }
});

References

• ↗ https://www.php.net/manual/en/function.set-error-handler.php

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