0 == *计算结果为真，0 === *计算为假

Question

This morning while editing our CRON editor that suddenly failed to run, I found an oddity that I can't really explain. Hoping someone here can enlighten me.

Supposing someone can answer this without any context, my question is essentially "Why does 0 == * evaluate to true?"

Details

We store the details of all active CRON jobs in a MySQL table. The type for fields Minutes, Hours, Days and Months is varchar with default '*'. When editing a CRON job, we have a select that should should have selected the current value from a fetched row, labeled below as $CRON and an example of which follows.

Array ( [CRONID] => 10 [Minutes] => 21 [Hours] => 6 [Days] => * [Months] => * [Weekday] => * [File] => CRON_Prop_44.php [Status] => 1 [CreateBy] => 128 [CreateDate] => 2014-02-20 [ChangeBy] => 128 [ChangeDate] => 2014-02-20 )

The Broken Select

The following would produce a select with Sunday as the selected option. Inspector reveals that both option value="*" and option value="0" are selected.

    <select name="Weekday">
    <option value="*" <?php if ($CRON['Weekday'] == "*" ) { echo "selected";} ?> >* - Every day</option>
<?php
$x = 0;
while ($x < 7) {
    switch ($x) {
        case 0:
        $dayname = "Sunday";
        break;
        case 1:
        $dayname = "Monday";
        break;
        case 2:
        $dayname = "Tuesday";
        break;
        case 3:
        $dayname = "Wednesday";
        break;
        case 4:
        $dayname = "Thursday";
        break;
        case 5:
        $dayname = "Friday";
        break;
        case 6:
        $dayname = "Saturday";
        break;
    }
?>
    <option value="<?php echo $x; ?><?php if ($CRON['Weekday'] == $x ) { echo "selected";} ?> ><?php echo $dayname; ?></option>
<?php
    $x++;
}?>

The fact that Sunday is also selected leads me to believe that 0 == * is true. I can get the above working as expected by adding the === comparison operator instead of ==, but I don't understand why. Is it something with the switch? The option value="0"? Or, as the title suggests, that 0 == * is true?

The (Sort of) Functioning Select

I thought the previous person's code above was a bit messy, so I'm currently doing the following. Still, changing the === to == results in only Sunday being selected.

<select name="Weekday">
    <option value="*" <?php if ($CRON['Weekday'] == "*" ) { echo "selected";} ?> >* - Every day</option>
<?php
    $days = array("Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday");
    $x = 0;
    while ($x < 7) {
        echo '<option value="' .  $x . '" ' . (($CRON['Weekday'] == $x) ? 'selected' : "") . '>' . $days[$x] . '</option>';
        $x++;       
    }
?>
</select>

If you've lasted to the end of this post, please tell me what I am missing?

Answer 1

The string '*' evaluates to true because that's how strings should evaluate according to the contract of casting to boolean: http://www.php.net/manual/en/language.types.boolean.php

So there should be no wondering.

The a === b does not do any type coercion, which means that to be the same, a and b have to be:

• of the same type (integer/number or string)
• bear the same value

Now, an integer is not a string, so 0 === '*' evaluates to false.