PHP mysqli is NOT trapping some errors when calling stored procedure

Can you take a look at the issue that I've described at https://stackoverflow.com/questions/54643704/php-mysqli-is-not-trapping-some-errors-when-calling-stored-procedure and tell me what I am missing in my PHP code ? To summarize: I am calling a MySQL stored procedure from PHP that does some DMLs (using mysqli). But the problem is that PHP is not able to get intimated of some database error conditions raised by the stored proc (which I know are raised, bcoz they do get flagged when invoked from a mysql client) I've listed the relevant code fragments there. Am I not using mysqli correctly? Thanks, Venkat