this.state = {
    validating: false
}

render() {
    return (
        <Container>
            <Content>
                <Form>
                    <Item floatingLabel>
                        <Label>Email</Label>
                        <Input onChangeText={(text) => this.setState({email:text})} />
                    </Item>
                    <Item floatingLabel last>
                        <Label>Password</Label>
                        <Input secureTextEntry onChangeText={(text) => this.setState({password:text})} />
                    </Item>
                    <Button block success style={{ marginTop: 50 }} onPress={() => {
                        if( this.state.email && this.state.password ){
                            this.validate();

                        }
                    }} >
                        <Text>Authenticate</Text>
                    </Button>
                </Form>
            </Content>
        </Container>
    )
}

<?php

require_once('wp-load.php');

$response = array(
    'data'      => array(),
    'msg'       => 'Invalid email or password',
    'status'    => false
);

/* Sanitize all received posts */
foreach($_POST as $k => $value){
    $_POST[$k] = sanitize_text_field($value);
}

/**
 * Login Method
 *
 */
if( isset( $_POST['type'] ) &&  $_POST['type'] == 'login' ){

    /* Get user data */
    $user = get_user_by( 'email', $_POST['email'] );

    if ( $user ){
        $password_check = wp_check_password( $_POST['password'], $user->user_pass, $user->ID );

        if ( $password_check ){
            /* Generate a unique auth token */
            $token = MY_RANDOM_CODE_GENERATOR( 30 );

            /* Store / Update auth token in the database */
            if( update_user_meta( $user->ID, 'auth_token', $token ) ){

                /* Return generated token and user ID*/
                $response['status'] = true;
                $response['data'] = array(
                    'auth_token'    =>  $token,
                    'user_id'       =>  $user->ID,
                    'user_login'    =>  $user->user_login
                );
                $response['msg'] = 'Successfully Authenticated';
            }
        }
    }
}

validate(){
    this.setState({ validating: true });

    let formData = new FormData();
    formData.append('type', 'login');
    formData.append('email', this.state.email);
    formData.append('password', this.state.password);

    return fetch('http://example.com/authentication.php', {
        method: 'POST',
        body: formData
    })
        .then((response) => response.json())
        .then((responseJson) => {
            let data = responseJson.data;

            if (this.saveToStorage(data)){
                this.setState({
                    validating: false
                });

                /* Redirect to accounts page */
                Actions.pageAccount();
            } else {
                console.log('Failed to store auth');
            }
        })
        .catch((error) => {
            console.error(error);
        });
}

async saveToStorage(userData){
    if (userData) {
        await AsyncStorage.setItem('user', JSON.stringify({
                isLoggedIn: true,
                authToken: userData.auth_token,
                id: userData.user_id,
                name: userData.user_login
            })
        );
        return true;
    }

    return false;
}

async logout(){
    await AsyncStorage.removeItem('user');

    // Add a method that will delete user_meta token of the user from the server.
    // await deleteUserMetaToken(PARAM_USER_ID);

    /* Redirect to the login page */
    Actions.pageLogin();
}