使用字符串变量时，匿名函数中的字符串比较失败

I'm making an API call to get a multidimensional array. I'm then using array_filter to try to get the specific array that has a specific end date. The code I'm using is below:

$api_call = "287/terms";

$terms_json = curl_exec(makeAPIConnection($api_call));
$all_terms = json_decode($terms_json);

if(!is_array($all_terms)) { return NULL; }

// Getting current date and formatting for comparison
$current_date = date_create('2017-05-25');
$date_formatted = date_format($current_date, 'Y-m-d');

// Getting the current term
$current_term = array_filter($all_terms, function($a) {
    if(substr($a->EndDate, 0, 10) === $date_formatted) {
        return true;
    }
    return false;
});

echo "<pre>";
var_dump($date_formatted) . "<br";
var_dump($current_term) . "<br";
echo "</pre>";

That code returns this.

string(10) "2017-05-25"
array(0) {
}

If I instead use a string literal in the anonymous function...

$current_term = array_filter($all_terms, function($a) {
    if(substr($a->EndDate, 0, 10) === '2017-05-25') {
        return true;
    }
    return false;
});

I get this.

string(10) "2017-05-25"
array(1) {
  [3]=>
  object(stdClass)#4 (7) {
    ["AcadSessionId"]=>
    int(287)
    ["Code"]=>
    string(4) "Qtr4"
    ["Description"]=>
    string(20) "Quarter 4/Semester 2"
    ["EndDate"]=>
    string(19) "2017-05-25T00:00:00"
    ["Id"]=>
    int(729)
    ["Name"]=>
    string(20) "Quarter 4/Semester 2"
    ["StartDate"]=>
    string(19) "2017-03-13T00:00:00"
  }
}

Can anyone tell me why using the string variable is failing and using the string literal is working?

Don't disable error reporting or else you would have gotten a notice that $date_formatted was undefined.

$date_formatted does not exist in the context of your anonymous function. You can inherit variables from the parent scope by using use:

$current_term = array_filter($all_terms, function($a) use ($date_formatted) {
    if(substr($a->EndDate, 0, 10) === $date_formatted) {
        return true;
    }
    return false;
});

More information can be found at http://php.net/manual/en/functions.anonymous.php