PHP为什么必须在循环之前预先定义变量？

Question

Why do PHP variables need to be pre-defined before a loop in order to use it inside of (and subsequently outside of) the loop?

Assuming $results is an array of integers.

foreach ($results as $key=>$value)
{
    $a = $value[0];
    echo "<BR>".$a;
}
echo "<BR>".$a;

would output something like:

1
5
7
25
undefined variable $a

and I would not be able to use the variable $a (without initializing it somehow) outside of that loop because it is "undefined".

Whereas

$a;
foreach ($results as $key=>$value)
{
    $a = $value[0];
    echo "<BR>".$a;
}
echo "<BR>".$a;

would output something like:

1
5
7
25
25

And I would be able to use $a anywhere outside.

Seems weird to me that it almost acts like a variable scope issue.

Answer 1

Because you are trying to concatenate a string to something that's not a string (or at least not yet) by doing:

$source .= '<source type="' . $format . '" src="' . $src . '">';
      //^ this dot means concat.

And in your second example, you are resetting your variable to an empty string in each iteration, the declaration should be outside, before the loop instead:

$source2 = '';
foreach( $formats as $format => $src ){   
    if ( !empty( $src ) ) {
        $source2 .= '<source type="' . $format . '" src="' . $src . '">';
    }
}
echo $source2;

As @drew010 stated in his comment, the variable will be actually created by php but you will get the notice displayed (if you have your php settings so that they will be shown).