{"created_at":"Sun Jun 11 07:50:06 +0000 2017","id":873809551614660608,"id_str":"873809551614660608",

$s = '
{"created_at":"Sun Jun 11 07:50:06 +0000 2017","id":873809551614660608,"id_str":"873809551614660608",
';

วิธี

$string = '{"created_at":"Sun Jun 11 07:50:06 +0000 2017","id":873809551614660608,"id_str":"873809551614660608"';
echo $string.'<br>'."\n";

preg_match('#[\'|"]id[\'|"][\s]*:[\s]*(?<id>\d+)#i', $string, $matches);
echo '<pre>'.print_r($matches, true).'</pre>'."\n";

จะได้

Array
(
[0] => "id":873809551614660608
[id] => 873809551614660608
[1] => 873809551614660608
)

* id คลุมด้วย ” หรือ ‘ ก็ได้ (ใช้ [\’|”])
* [\s]*:[\s]* หมายถึง หลังและก่อน : มีหรือไม่มี space ก็ได้
* ?<id> คือระบุลงไปว่าถ้าเจอให้จับมันยัดใส่ array key “id”
* i ด้านท้าย pattern หมายถึงตัวใหญ่เล็กได้หมด

—
ถ้าเป็นแบบนี้ละ

[id] => 874536447386177536 [id_str] => 874536447386177536 [text] =>

วิธี

$string = '{"created_at":"Sun Jun 11 07:50:06 +0000 2017","id":873809551614660608,"id_str":"873809551614660608"';
preg_match_all('#[\'|"](\w+)[\'|"][\s]*:[\s]*["]?([a-zA-Z0-9\:\-_+\s]*)["]?#i', $string, $matches);
echo '<pre>'.print_r($matches, true).'</pre>'."\n";

จะได้

Array
(
[0] => Array
(
[0] => "created_at":"Sun Jun 11 07:50:06 +0000 2017"
[1] => "id":873809551614660608
[2] => "id_str":"873809551614660608"
)

[1] => Array
(
[0] => created_at
[1] => id
[2] => id_str
)

[2] => Array
(
[0] => Sun Jun 11 07:50:06 +0000 2017
[1] => 873809551614660608
[2] => 873809551614660608
)

)