Php: what's the difference between $var and &$

问题:

What is the difference between

foreach ($my_array as $my_value) {
}

And:

foreach ($my_array as &$my_value) {
}

?

May I ask you to give me two real-world examples of when to use one and when you use the other?

回答1:

The first example creates a copy of the value, whereas the second uses a reference to the original value. So after the first foreach runs, the original array is still untouched. After the second foreach the original array could have been modified since it was handled by reference.

Some native PHP functions already work this way, such as shuffle() which rearranges the contents of your array. You'll notice that this function doesn't return an array, you just call it:

$myArray = array('foo', 'bar', 'fizz', 'buzz');
shuffle( $myArray );
// $myArray is now shuffled

And it works its magic since it works with the array by reference rather than creating a copy of it.

Then there are functions that don't pass anything by reference but rather deal with a copy of the original value, such as ucwords() which returns the new resulting string:

$myString = "hello world";
$myString = ucwords( $myString );
// $myString is now capitalized

See Passing by Reference.

回答2:

Jonathan's answers describes it very well. Just for completeness, here are your two examples:

1. Just reading values:

   $my_array = range(0,3);
   foreach ($my_array as $my_value) {
       echo $my_value . PHP_EOL;
   }
   

2. Adding some number to each element (thus modifying each value):

   foreach ($my_array as &$my_value) {
       $my_value += 42;
   }
   

   If you don't use &$my_value, then the addition won't have any effect on $my_array. But you could write the same not using references:

   foreach($my_array as $key=>$value) {
       $my_array[$key] = $value + 42;
   }
   

   The difference is that we are accessing/changing the original value directly with $my_array[$key].

DEMO

回答3:

When you prefix a variable with an ampersand, you’re creating a “reference.” PHP references are like shortcuts or symlinks on your computer. You can create a pointer variable that is just another name for the same data.

I dont see a big difference in using these, except that you DONT COPY a variable saving memory. When you are passing variables you can just pass the reference and the reference points to the original object.

回答4:

A real world example for & use is when you need to change the content of an array with very few lines of code

foreach($arrFeed as &$objFeed)
  $objFeed['externalSrc'] = convertToLocalImage($objFeed['externalSrc']);

回答5:

I only use references when i have to read csv files to know what is the delimitier. Just take the first line:

$handle = fopen($file, "r");
$firstLine = fgets($handle);
fclose($handle);

Then with this possible delimitiers

$delimiters = array(';' => 0,',' => 0,"\t" => 0,"|" => 0);

Count by reference what is the most used

foreach ($delimiters as $delimiter => &$count) {
    $count = count(str_getcsv($firstLine, $delimiter));
}
$delimiter =  array_search(max($delimiters), $delimiters);