It helps to put a space between code and the closing tags, instead of:

PHP: Select all

<?php $rand_str=random_string(5);?> 
<?php echo $rand_str?>

Use:

PHP: Select all

<?php $rand_str=random_string(5); ?> 
<?php echo $rand_str; ?>

Cloud is right though,this is not javascript so there error is probably caused by mixing html and php incorrectly.