如何在php中将单词转换为方程式

<?php
function wordToEquation($word) {
    $equation = '';

    switch ($word) {
        case 'add':
            $equation = '+';
            break;
        case 'subtract':
            $equation = '-';
            break;
        case 'multiply':
            $equation = '*';
            break;
        case 'divide':
            $equation = '/';
            break;
        default:
            $equation = 'Invalid word';
            break;
    }

    return $equation;
}

$word = 'add';
$equation = wordToEquation($word);
echo $equation;  // 输出 "+"
?>