//selecting user based on his personal ID number
if(filter_var($var, FILTER_VALIDATE_INT)) $sql = "select * from user where JMBG='$var'";
//selecting user based on his email
else if(filter_var($var, FILTER_VALIDATE_EMAIL)) $sql = "select * from user where email='$var'";
//selecting user based on his username
else $sql = "select * from user where username='$var'";

This code generates SQL depending on semantic type of  variable from user input.

php>5.2

Using PHP's filter_var to validate email or number

PHP