php中字符串和数字比较时自动转换类型的问题

字符串会被转化为0，请看以下说明：

If you compare an integer with a string, the string is converted to a number. If you compare two numerical strings, they are compared as integers. These rules also apply to the switch statement.

<?php
var_dump(0 == "a"); // 0 == 0 -> true
var_dump("1" == "01"); // 1 == 1 -> true
var_dump("1" == "1e0"); // 1 == 1 -> true

switch ("a") {
case 0:
    echo "0";
    break;
case "a": // never reached because "a" is already matched with 0
    echo "a";
    break;
}
?>

参见 http://cn2.php.net/manual/en/language.operators.comparison.php