Alternate line colors - understanding a code provided

In an example like this:

$c = true; // Let's not forget to initializ开发者_JAVA技巧e our variables, shall we?
foreach($posts as $post)
    echo '<div'.(($c = !$c)?' class="odd"':'').">$post</div>";

I would like to understand how this works.

What are we trying to do with this example? Do an alternate div row by changing true to false and false to true?

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Yes.

$c = !$c assigns the opposite value of $c to itself. The variable is then evaluated after the assignment.

This results in a constantly changing value between true and false.

This codes takes advantage of the foreach loop. If you have a normal for loop, you could use the counter variable instead:

for($i = 0, $l = count($posts); $i < $l; $i++) {
    echo '<div'.(($i % 2)?' class="odd"':'').">{$posts[$i]}</div>";
}

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If you assign meaningful names to your variables and you're generous with white-space, the code is normally easier to understand:

<?php

$odd = true;
foreach($posts as $post){
    echo '<div' . ( $odd ? ' class="odd"' : '' ) . ">$post</div>";
    $odd = !$odd;
}

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There is a bunch of trickery going on in a very short space here. You could split the inside of the loop up into three lines:

$c = !$c; // invert c
$class_part = $c ? ' class="odd"':''; // if c is true, class is odd.
echo "<div$class_part>$post</div>"; // print the <div> with or without the class
                                    // depending on the iteration

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Yes.

$c = true;
$not_c = !$c; // $not_c is now false
$c = !$c;     // same as above, but assigning the result to $c. So $c is now false
$c = !$c;     // $c is now true again

The snippet you provided could be rewritten (and arguably made more clear) like so:

$c = true;
foreach ($posts as $post) {
    $c = !$c;
    echo '<div' . ($c ? ' class="odd"' : '') . ">$post</div>";
}

The $c ? ... : ... syntax is using the ternary operator. It's kind of like a short-hand if statement. For example, true ? "a" : "b" evaluates to "a".

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Assignments in PHP return the newly assigned value. So $c = !$c returns true when $c was false; false when $c was true.

The ternary operator ( ? : ) evaluates the part before ':' when the condition before '?' is true, otherwise the part after the ':'. So it outputs the text either before or after the ':'.

As others stated, it's probably better to write this in a more understandable way.