PHP Manual . ... Next . PHP/FI 2.0 used the left side of expressions to determine what type the result should be. ... a[0]=5; $a[1]=7; $key = key($a); while ("" != $key) { echo "$keyn"; next($a); } . ... The reason is that in PHP 2.0, because the left argument's type was string, a string comparison was made, and indeed "" does not equal "0" , and the loop went through. In PHP 3.0, when a string is compared with an integer, an integer comparison is made (the string is converted to an integer). ...