getting the even index of a string - php

Question

hey guys im trying to get the even indexes of a string from the db then save them in a variable then echo. but my codes seems doesnt work. please help. here it is
require_once('DBconnect.php');
$school_id = '1';
$section_id = '39';
$select_pk = "SELECT * FROM section
WHERE school_id = '$school_id'
AND section_id = '$section_id'";
$query = mysql_query($select_pk) or die (mysql_error());
while ($row = mysql_fetch_assoc($query)) {
$public_key = $row['public_key'];
}
if ($public_key) {
$leng_public_key = strlen($public_key);
$priv_key_extract = "";
$array_pki = array();
for ($i=0; $i <=$leng_public_key-1 ; $i++) {
array_push($array_pki,$public_key[$i]);
}
foreach ($array_pki as $key => $value) {
if($key % 2 == 0) {
$priv_key_extract += $public_key[$key];
} else {
$priv_key_extract ="haiiizzz";
}
}
}
echo $priv_key_extract;
as you can see im trying to use modulo 2 to see if the index is even.

Answer 1

I have updated your code as below, it will work now :
<?php
$public_key = 'A0L8V1I5N9';
if ($public_key) {
$leng_public_key = strlen($public_key);
$priv_key_extract = "";
$array_pki = array();
for ($i=0; $i <=$leng_public_key-1 ; $i++) {
array_push($array_pki,$public_key[$i]);
}
foreach ($array_pki as $key => $value) {
//Changed condition below $key % 2 ==0 => replaced with $key % 2 == 1
if($key % 2 == 1) {
// Changed concatenation operator , += replaced with .=
$priv_key_extract .= $public_key[$key];
} /*else {
//Commented this as it is getting overwritten
$priv_key_extract ="haiiizzz";
}*/
}
}
echo $priv_key_extract;
?>

Answer 2

Try this function
function extractKey($key) {
if (empty($key) || !is_string($key)) return '';
$pkey = '';
for ($i=0;$i<strlen($key);$i++) {
if ($i % 2 == 0) {
$pkey .= $key[$i];
}
}
return $pkey;
}
echo extractKey('12345678'); # => 1357