Run JavaScript validation script on form submit in plugin

Two issues –

validateForm is an existing javascript function in WordPress core, change it to something else. Always prefix things with a unique string to prevent these problems.

Your onsubmit needs return added: onsubmit="return your_function()"

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)