Cant insert string into mysql query

Question

I'm trying to make a login page in PHP, and I'm trying to construct the query here:

$q = 'SELECT * FROM users WHERE userid="'+$username+'"';

When I echo it out with

echo $q

I get 0. When I do

$q = 'SELECT * FROM users WHERE userid="'+"test"+'"';

I get 0. When I do

$q = 'SELECT * FROM users WHERE userid="michael"';

I get my expected result of the string being printed out

Answer 1

Use a . for concatenation, also don't forget to clean the data to prevent mysql injection.

$user_id = 'test';
$q = 'SELECT * FROM users WHERE userid="' . $user_id . '"';