删除重复链接

I wanna to crawl pdf links. But some links that I get are double. How to remove the one of the double links? Thank you :)

<?php
<include 'simple_html_dom.php';
$url = 'http://scholar.google.com/scholar?hl=en&q=data+mining&btnG=&as_sdt=1%2C5&as_sdtp=';
$html = file_get_html($url) or die ('invalid url');
foreach($html->find('a') as $e) {
    $link= $e->href;
    if (preg_match('/\.pdf$/i', $link)) {
       print_r($link);
    }
}
?>

Put the links in an array and then use array_unique()

foreach($html->find('a') as $e) {
    $link= $e->href;
    if (preg_match('/\.pdf$/i', $link)) {
       $links[] = $link;
    }
}
$links = array_unique( $links );

$url = 'http://scholar.google.com/scholar?hl=en&q=data+mining&btnG=&as_sdt=1%2C5&as_sdtp=';
$html = file_get_html($url) or die ('invalid url');
$arr = array();
foreach($html->find('a') as $e) {
    $link= $e->href;
    if(strtolower(substr($link, strrpos($link, '.'))) === '.pdf')
       $arr[] = $link;
}
array_unique($arr);
print_r($arr);

1. I'd recommend to use strings functions to get the the extension - it's lighter.
2. You can store your links in array, and then you use array_unique() function.