In PHP, why does the elseif statement in Instagram access token function never return true?

This code requests an access token from Instagram, passes it into a function to get a long-lived access token and with this, gets a user's information and posts.

It does get the post, but on refresh I'm getting the error message { ["error_type"]=> string(14) "OAuthException" ["code"]=> int(400) ["error_message"]=> string(37) "This authorization code has been used" }.

I think that's because if ( $params['access_token'] ) never fires.

But I'm not sure why when in the elseif statement ( $params['get_code'] ) the variable for the access token gets updated.

Said elseif statement only fires if I set a variable and pass in a token, like $accessToken = 'ACCESS-TOKEN';. But since even the long-lived token expires after 60 days, I'm trying to find a solution that detects in how many seconds expiration is due and based on this, retrieves a new token. But to make this work, it first needs to recognize that there is a token even when a user refreshes the page or returns to it later...

This code gets the tokens:

    private function _setUserInstagramAccessToken( $params ) {

        if ( $params['access_token'] ) { // this never fires

            // we have an access token
            $this->_userAccessToken = $params['access_token'];
            $this->hasUserAccessToken = true;
            $this->userId = $params['user_id'];

            echo "access token available!";

        } elseif ( $params['get_code'] ) {  // this always fires

            echo "no access token found, go get one";

            // try to get access token and user id, then update respective vars
            $userAccessTokenResponse = $this->_getUserAccessToken();
            $this->_userAccessToken = $userAccessTokenResponse['access_token'];  //store access token
            $this->hasUserAccessToken = true;
            $this->userId = $userAccessTokenResponse['user_id']; //store user id

            // get long lived access token
            $longLivedAccessTokenResponse = $this->_getLongLivedUserAccessToken();
            $this->_userAccessToken = $longLivedAccessTokenResponse['access_token'];
            $this->_userAccessTokenExpires = $longLivedAccessTokenResponse['expires_in'];
        }
    }


    private function _getUserAccessToken() {
        $params = array(
            'endpoint_url' => $this->_apiBaseUrl . 'oauth/access_token',
            'type' => 'POST',
            'url_params' => array(
                'app_id' => $this->_appId,
                'app_secret' => $this->_appSecret,
                'grant_type' => 'authorization_code',
                'redirect_uri' => $this->_redirectUrl,
                'code' => $this->_getCode
            )
        );

        $response = $this->_makeApiCall( $params );
        return $response;
    }



    private function _getLongLivedUserAccessToken() {
        $params = array(
            'endpoint_url' => $this->_graphBaseUrl . 'access_token',
            'type' => 'GET',
            'url_params' => array(
                'client_secret' => $this->_appSecret,
                'grant_type' => 'ig_exchange_token',
            )
        );

        $response = $this->_makeApiCall( $params );
        return $response;
    }

This function makes the API call:

private function _makeApiCall( $params ) {
    $ch = curl_init();

    $endpoint = $params['endpoint_url'];

    if ( 'POST' == $params['type'] ) { 

        // post request
        curl_setopt( $ch, CURLOPT_POSTFIELDS, http_build_query( $params['url_params'] ) );
        curl_setopt( $ch, CURLOPT_POST, 1 );

    } elseif ( 'GET' == $params['type'] && !$params['url_params']['paging'] ) { 

        // get request
        $params['url_params']['access_token'] = $this->_userAccessToken;

        //add params to endpoint
        $endpoint .= '?' . http_build_query( $params['url_params'] );
    }

    // general curl options
    curl_setopt( $ch, CURLOPT_URL, $endpoint );

    curl_setopt( $ch, CURLOPT_SSL_VERIFYHOST, false );
    curl_setopt( $ch, CURLOPT_SSL_VERIFYPEER, false );
    curl_setopt( $ch, CURLOPT_RETURNTRANSFER, true );

    $response = curl_exec( $ch );

    curl_close( $ch );

    $responseArray = json_decode( $response, true );

    if ( isset( $responseArray['error_type'] ) ) {
        var_dump( $responseArray );
        die();
    } else {
        return $responseArray;
    }
}

Above my HTML, I have defined:

<?php    
    require_once ('ig_conf.php');

    // if there is an auth 'code' in the URL, get it and pass it along
    // if there is NO auth 'code' in the URL, leave blank and init auth process

    $params = array(
        'get_code' => isset( $_GET['code'] ) ? $_GET['code'] : '',
        'access_token' => $accessToken,
        'user_id' => '123456789' // insert actual user ID here to make code work
    );

    // init class for handling instagram, from ig_config.php
    // pass on 'get_code' as $params into instagram_basic_display_api
    // ...this gets the authorization url

    $ig = new instagram_basic_display_api( $params );
?>

Help is much appreciated. If more code is required, I can provide.