PHP Warning : A non-numeric value encountered

Post meta values are stored as strings in the database, so you get only strings back. If you want to use these value in mathematical operations, you have to cast the value to a numeric type, meaning you write the desired type in parentheses in front of the variable or the function call.

Generic example:

$number = (int) get_post_meta($post_id, "key_identifier", true);

And then you can use the variable with a + sign.

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