How can an elephant count for nothing?

While PHP has a lot of nice features, it also has some traps which I am forever falling into. One which I find particularly hard to understand is how mixed mode comparisons work. For instance $string = 'elephant'; If($string == 0) returns true; If($string != 0) returns false; If($string === 0) returns false; I know that in this case I should use 'If($string == '')', but I still manage to forget. Can anyone explain clearly why comparing a string with zero gives this apparently anomalous result?