Delete query deletes from all usesrs

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First of all your script has a problem with the usage of $num. It is an array fetched using a PDO method, and you are checking if its value equals to 0? An array cannot equal to 0 unless it contains only one element(which is considered a string instead), the statement will always evaluate to be false...

On the other hand, you are using the delete() method incorrectly. It does not accept an empty array as its second argument, so get rid of it. I am surprised that you do not receive an error message for the way you use delete().