The use of explode and implode in PHP

$str = "hi, jason, world";
print_r(explode(',', $str));
print_r(explode(',', $str, 2));

Array
(
[0] => hi
[1] => jason
[2] => world
)
Array
(
[0] => hi
[1] => jason, world
)

 $a='331612_1,331495_1,331461_2,331559_1,333080_1,333555_1,331448_1,331618_1,333388_1,33
 3408_1,327937_1,331615_1,331499_1';
 $arr=explode(',',$a);
 $sum=0;
 foreach ($arr as $value) {
     $ary=explode('_', $value);
     $sum+=$ary[1];
 }
 print_r($sum);

<?php
$array  = array( &#39;lastname&#39; ,  &#39;email&#39; ,  &#39;phone&#39; );
 $comma_separated  =  implode ( "," ,  $array );
echo  $comma_separated ;  // lastname,email,phone