The issue: In general PHP is loosely typed.
<?php
$foo = "1";
$bar = $foo + 1;
var_dump($foo, $bar);
?>
Output
string(1) "1" int(2)
The normal comparison operators are loosely typed, too.
<?php
    $int = 1;
    $string = "1";
    $bool = true;
    var_dump($int == $string);
    var_dump($string == $bool);
    var_dump($int == $bool);
?>
Output
bool(true) bool(true) bool(true)
This can cause WTF situations
<?php
var_dump( '1' == '1.' );
?>
Output
bool(true)
This may lead to very common mistakes:
<?php
    function foo($answer) {
        if ($answer > 10) {
            return true;
        } else {
            return $answer;
        }
    }
    if (foo(11)) {
        echo "11 is greater 10<br />";
    }        
    if (foo(9)) {
        echo "9 is greater than 10<br />";
    }
?>
Output
11 is greater 10
9 is greater than 10