// in .htaccess
RewriteEngine on
# If a directory or a file exists, use it directly
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
# Otherwise forward it to index.php
RewriteRule . index.php


// in config/web.php
'components' => [
    // ...
    'urlManager' => [
        'class' => 'yii\web\UrlManager',
        // Hide index.php
        'showScriptName' => false,
        // Use pretty URLs
        'enablePrettyUrl' => true,
       'rules' => [
            '<alias:\w+>' => 'site/<alias>',
       ],
    ],
    // ...
],