This should do the trick:
<?php
$array = array( '79900',
'79040',
'79100',
'79990',
'79490',
'79290',
'79990');
$count = array_count_values($array);
echo "<pre>".print_r($array, true)."</pre>";
foreach($array as $val)
{
if($count[$val]>1) {
$output[] = "*".$val."*";
} else {
$output[] = $val;
}
}
echo "<pre>".print_r($output, true)."</pre>";
?>
Outputs:
Array
(
[0] => 79900
[1] => 79040
[2] => 79100
[3] => 79990
[4] => 79490
[5] => 79290
[6] => 79990
)
Array
(
[0] => 79900
[1] => 79040
[2] => 79100
[3] => *79990*
[4] => 79490
[5] => 79290
[6] => *79990*
)
Note: Your [0] isn't actually the same as [3] and [6], but I'm assuming this is just a typo
Let me know how you get on!