The question is the title alone, in PHP we can do:
var_dump((int) "!");
And receive int(0) .
And in MySQL we can do:
SELECT campo_int FROM tabela WHERE campo_int = "!"
And if that 'campo_int' is '0' it returns the record.
When you run this command:
var_dump((int) "!");
In fact it returns false since it could not cast for int, if you do this for example:
var_dump((int) "qualquerCoisaString");
It will return 0 in the same way, since it can not convert to int, and in mysql it is the same, if you do so:
SELECT campo_int FROM tabela WHERE campo_int = "qualquerCoisaString"
It will return the values of the field_int that has the value 0, since its expression returned 0 (false).