validar consulta sql

What exactly are you trying to do? Are you trying to find out IF any records exist when querying your database and IF they don't return a message or do something else?

Something like below where the number of rows are returned from the database query into a variable  - $numRows

$conn = new mysqli('localhost' , 'username' , 'password' , 'databaseName');

$sqlQuery = $conn->query("SELECT * FROM tableName WHERE id= '3'") or die($conn->error);
$sqlQuery = $sqlQuery->fetch_assoc();
$numRows = $sqlQuery->num_rows;

If no results are found:

if($numRows == 0) {

Do Something

}

else {

Do Something Else

}

Don't blame the software.  Answers are contained in your code.