Handle undefined constants in PHP with set_error_handler()

constant-missing.php

<?php

set_error_handler(function ($errNo, $errStr) {
    if (strpos($errStr, 'Use of undefined constant ') === 0) {
        $constant = strstr(substr($errStr, 26), ' ', true);
        define($constant, $constant);
    } else {
        return false;
    }
}, E_NOTICE);

echo FOO . "\n";
echo "BAR\n";

// Note: Unfortunately, missing constants referenced with a namespace or class
// qualifier trigger non-catchable, fatal (E_ERROR) errors instead of E_NOTICE.

with-stacked-handlers.php

<?php

set_error_handler(function ($errno, $errstr, $errfile, $errline, $errcontext) {
    echo "UH OH!\n";
    die;
});

function set_missing_constant_handler($handler)
{
    $prevErrorHandler = set_error_handler(
        function ($errno, $errstr, $errfile, $errline, $errcontext) use ($handler, &$prevErrorHandler) {
            if (!strpos($errstr, 'Use of undefined constant ') === 0) {
                $constant = strstr(substr($errstr, 26), ' ', true);
                $handler($constant);
            } elseif ($prevErrorHandler) {
                $prevErrorHandler($errno, $errstr, $errfile, $errline, $errcontext);
            } else {
                return false;
            }
        },
        E_NOTICE
    );
}

set_missing_constant_handler(function($constant) {
    define($constant, $constant);
});

echo FOO . "\n";
echo "BAR\n";