Source

File: wp-includes/class-wp-user.php

public static function get_data_by( $field, $value ) {
		global $wpdb;

		// 'ID' is an alias of 'id'.
		if ( 'ID' === $field ) {
			$field = 'id';
		}

		if ( 'id' === $field ) {
			// Make sure the value is numeric to avoid casting objects, for example,
			// to int 1.
			if ( ! is_numeric( $value ) ) {
				return false;
			}
			$value = intval( $value );
			if ( $value < 1 ) {
				return false;
			}
		} else {
			$value = trim( $value );
		}

		if ( ! $value ) {
			return false;
		}

		switch ( $field ) {
			case 'id':
				$user_id  = $value;
				$db_field = 'ID';
				break;
			case 'slug':
				$user_id  = wp_cache_get( $value, 'userslugs' );
				$db_field = 'user_nicename';
				break;
			case 'email':
				$user_id  = wp_cache_get( $value, 'useremail' );
				$db_field = 'user_email';
				break;
			case 'login':
				$value    = sanitize_user( $value );
				$user_id  = wp_cache_get( $value, 'userlogins' );
				$db_field = 'user_login';
				break;
			default:
				return false;
		}

		if ( false !== $user_id ) {
			$user = wp_cache_get( $user_id, 'users' );
			if ( $user ) {
				return $user;
			}
		}

		$user = $wpdb->get_row(
			$wpdb->prepare(
				"SELECT * FROM $wpdb->users WHERE $db_field = %s LIMIT 1",
				$value
			)
		);
		if ( ! $user ) {
			return false;
		}

		update_user_caches( $user );

		return $user;
	}