Handle undefined index error

gistfile1.php

<?php

set_error_handler('undefinedIndexHandler');

function undefinedIndexHandler($errno, $errstr, $errfile, $errline, $errcontext)
{
    if (!preg_match('/Undefined index/', $errstr)) {
        return;
    }
    
    $ouput = 'Maybe you should try one of these keys : ';

    foreach(array_keys($errcontext['a']) as $key) {
        $ouput .= sprintf(' "%s" ', $key);
    }

    printf($ouput);
}

$a=['foo' => 1, 'bar' => 2, 'baz' => 3];

$a['toto'];