<?php
 
$input = array (3, 6, -3, 5, -10, 3, 10, 1, 7, -1, -9, -8, 7, 7, -7, -2, -7);
function pairs($array=[]){
    $pairs = [];
    // counting the positive and negative of each number in the set
    foreach($array as $v){
        $av = abs($v);
        if(!isset($pairs[$av]))
            $pairs[$av] = [
                'pos' => 0,
                'neg' => 0
            ];
 
        ($v > 0) ? $pairs[$av]['pos']++ :  $pairs[$av]['neg']++;
    }
 
    $pair_count = 0;
    // getting the number of pairs for each number, based on those counts
    foreach($pairs as $pair){
        $pair_count += min($pair['pos'],$pair['neg']);
    }
    return $pair_count;
}
echo pairs($input);

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