php数组语法$ {让我很困惑

I create a $values array and then extract the elements into local scope.

 $values['status'.$i] = $newStatus[$i];
 extract($values);

When I render an html page. I'm using the following

 <?php if(${'status'.$i} == 'OUT'){ ?>

but am confused by what the ${ is doing and why $status.$i won't resolve

$status.$i means

take value of $status variable and concatenate it with value of $i variable.

${'status'.$i} means

take value of $i variable, append id to 'status' string and take value of a variable 'status'.$i

Example:

With $i equals '2' and $status equals 'someStatus':

$status.$i evaluated to 'someStatus' . '2', which is 'someStatus2'
${'status'.$i} evaluated to ${'status'.'2'} which is $status2. And if $status2 is defined variable - you will get some value.

I wanted to add to the accepted answer with a suggested alternate way of achieving your goal.

Re-iterating the accepted answer...

Let's assume the following,

$status1 = 'A status';
$status = 'foo';
$i = 1;
$var_name = 'status1';

and then,

echo $status1; // A status
echo $status.$i; // foo1
echo ${'status'.$i}; // A status
echo ${"status$i"}; // A status
echo ${$var_name}; // A status

The string inside the curly brackets is resolved first, effectively resulting in ${'status1'} which is the same as $status1. This is a variable variable.

Read about variable variables - http://php.net/manual/en/language.variables.variable.php

An alternative solution

Multidimensional arrays are probably an easier way to manage your data.

For example, instead of somthing like

$values['status'.$i] = $newStatus[$i];

how about

$values['status'][$i] = $newStatus[$i];

Now we can use the data like,

extract($values);

if($status[$i] == 'OUT'){ 
   // do stuff
}

An alternative solution PLUS

You may even find that you can prepare your status array differently. I'm assuming you're using some sort of loop? If so, these are both equivalent,

for ($i=0; $i<count($newStatus); $i++){
    $values['status'][$i] = $newStatus[$i];
}

and,

$values['status'] = $newStatus;