Add a like button without a plugin in WordPress

like-it-enqueue.php

add_action( 'wp_enqueue_scripts', 'pt_like_it_scripts' );
function pt_like_it_scripts() {
    if( is_single() ) {
 
        wp_enqueue_style( 'like-it', trailingslashit( plugin_dir_url( __FILE__ ) ).'css/like-it.css' );
         
        if (!wp_script_is( 'jquery', 'enqueued' )) {
            wp_enqueue_script( 'jquery' );// Comment this line if you theme has already loaded jQuery
        }
    wp_enqueue_script( 'like-it', trailingslashit( plugin_dir_url( __FILE__ ) ).'js/like-it.js', array('jquery'), '1.0', true );
 
        wp_localize_script( 'like-it', 'likeit', array(
            'ajax_url' => admin_url( 'admin-ajax.php' )
        ));
    }
}

like-it-function.php

add_action( 'wp_ajax_nopriv_pt_like_it', 'pt_like_it' );
add_action( 'wp_ajax_pt_like_it', 'pt_like_it' );
function pt_like_it() {
 
    if ( ! wp_verify_nonce( $_REQUEST['nonce'], 'pt_like_it_nonce' ) || ! isset( $_REQUEST['nonce'] ) ) {
        exit( "No naughty business please" );
    }
 
    $likes = get_post_meta( $_REQUEST['post_id'], '_pt_likes', true );
    $likes = ( empty( $likes ) ) ? 0 : $likes;
    $new_likes = $likes + 1;
 
    update_post_meta( $_REQUEST['post_id'], '_pt_likes', $new_likes );
 
    if ( defined( 'DOING_AJAX' ) && DOING_AJAX ) {
        echo $new_likes;
        die();
    }
    else {
        wp_redirect( get_permalink( $_REQUEST['post_id'] ) );
        exit();
    }
}

like-it-style.css

.pt-like-it {
    display: inline-block;
    position: relative;
    height: 38px;
    line-height: 38px;
    padding: 0 16px;
    outline: 0;
    color: #57ad68;
    background: rgba(0,0,0,0);
    font-size: 14px;
    text-align: center;
    text-decoration: none;
    cursor: pointer;
    border: 1px solid #a9d8b2;
    vertical-align: bottom;
    white-space: nowrap;
    text-rendering: auto;
    -webkit-user-select: none;
    -moz-user-select: none;
    -ms-user-select: none;
    user-select: none;
    box-sizing: border-box;
    border-radius: 999em;
    letter-spacing: -0.02em;
    font-weight: 400;
    font-style: normal;
    text-rendering: optimizeLegibility;
    -webkit-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;
    -moz-font-feature-settings: "liga" on;
}
.pt-like-it:hover,
.pt-like-it:hover a {
    border-color: #57ad68;
    color: #468c54;
}
.pt-like-it > a.like-button {
    border: none;
}
.pt-like-it .like-button:before {
    font: normal normal normal 14px/1 FontAwesome;
    content: '\f087';
    margin-right: 3px;
}
.pt-like-it .like-count {
    padding-left: 10px;
}

like-it.js

jQuery( document ).on( 'click', '.pt-like-it', function() {
    var post_id = jQuery(this).find('.like-button').attr('data-id'),
        nonce = jQuery(this).find('.like-button').attr("data-nonce");
 
    jQuery.ajax({
        url : likeit.ajax_url,
        type : 'post',
        data : {
            action : 'pt_like_it',
            post_id : post_id,
            nonce : nonce
        },
        success : function( response ) {
            jQuery('#like-count-'+post_id).html( response );
        }
    });
      
    return false;
})

like-it.php

add_filter( 'the_content', 'like_it_button_html', 99 );
 
function like_it_button_html( $content ) {
    $like_text = '';
    if ( is_single() ) {
        $nonce = wp_create_nonce( 'pt_like_it_nonce' );
        $link = admin_url('admin-ajax.php?action=pt_like_it&post_id='.$post->ID.'&nonce='.$nonce);
        $likes = get_post_meta( get_the_ID(), '_pt_likes', true );
        $likes = ( empty( $likes ) ) ? 0 : $likes;
        $like_text = '
                    <div class="pt-like-it">
                        <a class="like-button" href="'.$link.'" data-id="' . get_the_ID() . '" data-nonce="' . $nonce . '">' . 
                        __( 'Like it' ) .
                        '</a>
                        <span id="like-count-'.get_the_ID().'" class="like-count">' . $likes . '</span>
                    </div>';
    }
    return $content . $like_text;
}