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I have the following two queries:

$s_name = $this->input->post('check_sur_name');
        $sql = "SELECT * FROM patient WHERE MATCH(s_name) AGAINST ('$s_name' IN BOOLEAN MODE ) or MATCH(f_name) AGAINST (''  IN BOOLEAN MODE)";
        $query = $this->db->query($sql);
        $query = $query->result_array();

        echo json_encode($query);

And this is the second one:

$sql = "SELECT * FROM patient WHERE MATCH(s_name) AGAINST ('Mugabe' IN BOOLEAN MODE ) or MATCH(f_name) AGAINST (''  IN BOOLEAN MODE)";
$query = $this->db->query($sql);
$query = $query->result_array();

echo json_encode($query);

The second script runs very well and gives me a feedback but the first one keeps failing, please advise what have I done wrong in the first script?