$var  = array();
function printTag($tags) {
        foreach($tags as $t) {
                echo $t['token'] . "/" . $t['tag'] .  " ";
                if($t['tag'] == 'NN' OR $t['tag']== 'JJ'){
                        array_push($var, $t['token'])   ;
                }
        }
        echo "n";
}

array_push() expects parameter 1 to be array, null given in /var/www/html/postag/poscall.php on line 9

<?php
// little helper function to print the results
include ("postag.php");
$var  = array();
function printTag($tags) {
        foreach($tags as $t) {
                echo $t['token'] . "/" . $t['tag'] .  " ";
                if($t['tag'] == 'NN' OR $t['tag']== 'JJ'){
                        array_push($var, $t['token'])   ;
                }
        }
        echo "n";
}

$tagger = new PosTagger('lexicon.txt');
$tags = $tagger->tag('The quick brown fox jumped over the lazy dog. this is really yummy and excellent pizza I have seen have really in love it it');
printTag($tags);
?>

function printTag($tags) {
   $var  = array();
   foreach($tags as $t) {
            echo $t['token'] . "/" . $t['tag'] .  " ";
            if($t['tag'] == 'NN' OR $t['tag']== 'JJ'){
                    array_push($var, $t['token'])   ;
            }
    }
    echo "n";
}

$var = array_reduce($tags, function(&$result, $t) {
    if (in_array($t['tag'], ['NN', 'JJ'])) {
        $result[] = $t['token'];
    }
    // you could do some output here as well
    return $result;
}, []);

function printTag(array $tags)
{
    $var = [];

    foreach($tags as $t) {
        // ...
    }

    return $var;
}

// ...
$var = printTag($tags);