How does PHP read if statements?

I have the following if statements in this order

if ( $number_of_figures_in_email < 6) {
       -- cut: gives false
}


if($number_of_emails > 0) {                                                                         
      -- cut: gives false
} 

if ( $number_of_emails == 0) {
   -- cut: gives true
}

The code behaves randomly. It sometimes goes to the third if clause and gives me a success, while sometimes to the one of the first two if clauses when the input variables are constant.

This suggests me that I cannot code only with if statements.