How does 'function' and 'use' and 'array_filter' work in PHP?

Question

I am familiar with creating a PHP function placed at the top of the .php file such as:

function my_little_function($parm1,$parm2) {
   if ($parms < $parm2) {
   return "yes";
   } else {
   return "no";
   }
}

Then call it like this:

$result = my_little_function("1","2");
echo "The answer is $result." . "\n";

I have some code, I didn't write it, which uses "function" and "use" together inside of a traditional use of a function like my_little_function above.

I'm puzzled by this and have some questions for you more experienced PHP developers. Here is part of the working PHP code I'm referring to:

$neededObject = array_filter($st_ny_trip->STOPS->STOP,function($e) use ($final_desired_dest,$connect_raw){return $e->NAME == $final_desired_dest && DateTime::createFromFormat("m/d/Y g:i:s a", $e->TIME) > $connect_raw;});

$e is not set in any part of the function or the rest of the program, so what is using $e? How does it get passed a value and how is it being used? There appears to be no name for this function, so I don't know how it is being called, how is that being done?

Is this creating a function, on-the-fly to be used and it gets re-generated each time this code gets called? If it's a function, why not create it outside of this function and call it?

I've also not used 'use' myself yet, so that's unfamiliar to me. I looked it up on php.net and it just looks like a way to assign a value to something, but I couldn't find any practical examples to demonstrate why it's needed and when it should be used.

I looked up array_filter and it says it's "Filters elements of an array using a callback function". I don't know what a call back function is. Is it referring to function($e)?

Should the above line of PHP code for $neededObject be formatted differently so it is easier to read?

Answer 1

Let's use array_map() to explain what's going on.

We want to duplicate the input of an array: so if the input is aa, the output would be aaaa.

So the normal way, would be to create a function and then pass it to array_map():

$array = range('a', 'e');

$new_array = array_map('duplicate', $array);
print_r($new_array);

function duplicate($string){
    return $string.$string;
}

Online demo

But what if you want to use this function only once ? Since PHP 5.3, there is something called anonymous functions, we use it like the following:

$array = range('a', 'e');

$new_array = array_map(function($string){
    return $string.$string;
}, $array);
print_r($new_array);

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Now, let's say for example you want to add a standard value from another variable. That's easy with global variables. But as we know, global variables are evil and should be avoided. We may use use():

$array = range('a', 'e');
$standard_value = ',';

$new_array = array_map(function($string)use($standard_value){
        // $standard_value becomes available inside the function
    return $string.$standard_value.$string;
}, $array);
print_r($new_array);

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use() can be become also useful if we use a reference to write to an external variable while looping:

$array = range('a', 'e');
$another_string = '';

$new_array = array_map(function($string)use(&$another_string){// note &
    $another_string .= $string.$string; // overwrite $another_string
    return $string.$string;
}, $array);
print_r($new_array);
echo PHP_EOL . $another_string;

Online demo