$rest = substr("56876578", 0, -4); //5687
$rest = substr("56876578", -4); //6758

Irelokeh:
Thanks bro but this echoes 6578 I kind off need a 67598 array

$part1 = substr("56876578", 0, -4); //5687
$part2 = substr("56876578", -4); //6758

$array = str_split($part1 ); // [5,6,8,7]
$array1 = str_split($part2 ); // [6,7,5,8]

Or 

$array =  str_split(substr("56876578", 0, -4)); // [5,6,8,7]
$array1 = str_split(substr("56876578", -4)); // [6,7,5,8]

jacob05:

ok.

 $part1 = substr("56876578", 0, -4); //5687
 $part2 = substr("56876578", -4); //6758

 $array = str_split($part1 ); // [5,6,8,7]
 $array1 = str_split($part2 ); // [6,7,5,8]

 Or 

 $array =  str_split(substr("56876578", 0, -4)); // [5,6,8,7]
 $array1 = str_split(substr("56876578", -4)); // [6,7,5,8]

 

Thanks mahn

Kidstell:
function imeiOdd ($imei){
$array=str_split ($imei);
for($i=0; $i <strlen($imei); $i++){
if ($i% 2){
unset ($array [$i]);
}
}
//return implode ($ array);
return array_values ($ array);
}

function imeiEven ($imei){
$array=str_split ($imei);
for($i=0; $i <strlen($imei); $i++){
if (! ($i% 2)){
unset ($array [$i]);
}
}
//return implode ($ array);
return array_values ($ array);
}


$imei='56876578';
$oddNoPositions=imeiOdd ($ imei);
//5867
$evenNoPositions=imeiEven ($ imei);
//6758

Just as u might expect there are easier but more confusing ways of doing this. like "array_walk" function.
if you want these functions to return string instead of numbered arrays then you should remove the comments.

babatope88:
OP try this Version more faster but little confusing.

<?php
$string = '56876578';

function imei($string)
{
$imei = '';
$len = 0.5 * strlen($string) + 1;
for ($i = 1; $i < $len; $i++) {
$j = $i << 1;
$imei .= $string[($j-1)];
}
return $imei;

}

print imei($string); //6758

Thanks guys you guys are awesome,nicee

<?php
$string = '56876578';
function imei($string)
{
    $imei = '';
    $len = 0.5 * strlen($string);
    for ($i = 1; $i <= $len; $i++) {
        $j = $i << 1;
        $imei .= $string[($j-1)];
    }
    return $imei;

}

print imei($string);//6758

DonSegmond:
$imei = implode(array_filter(str_split($string), function ($k) { return $k%2; }, ARRAY_FILTER_USE_KEY));

You can use the following, If you have up to date php at least 5.6.0

jacob05:
--------------------------------------
| PHP BENCHMARK SCRIPT |
--------------------------------------
Start : 2015-08-14 19:04:28
Server : localhost@?
PHP version : 5.5.12
Platform : WINNT
--------------------------------------
test_Kidstell_method : 0.016 sec.
test_babatope88_method : 0.002 sec.
--------------------------------------
Total time: : 0.018 sec.

--------------------------------------
| PHP BENCHMARK SCRIPT |
--------------------------------------
Start : 2015-08-14 19:04:28
Server : localhost@?
PHP version : 5.5.12
Platform : WINNT
--------------------------------------
test_Kidstell_method : 0.017 sec.
test_babatope88_method : 0.005 sec.
--------------------------------------
Total time: : 0.022 sec.
--------------------------------------------

Unable to test DonSegmond's Method because of my php version (requires 5.6.0)..