pluggable function in theme, to be overridden by plugin

Try something like this:

if ( !function_exists('get_field') && $_REQUEST['action'] != 'activate' )

This will prevent your function being defined for the run where a plugin is actually being activated.

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