Debian Stretch: replace php5 to 7.0

You can remove php5 by command
 # apt-get purge *php5*

Delete any configuration exist
# rm -R /etc/php5/

You need to know which php5 library you used and upgrade it to php7.0.
Note: php7.1 not available in Debian Stretch currently.

Show available php7.0
# apt-cache search 'php7.0'
php-amqp - AMQP extension for PHP
php-apcu - APC User Cache for PHP
php-all-dev - package depending on all supported PHP development packages
php-gearman - PHP wrapper to libgearman
php-geoip - GeoIP module for PHP
php-gmagick - Provides a wrapper to the GraphicsMagick library
php-gnupg - PHP wrapper around the gpgme library
php-igbinary - igbinary PHP serializer
php-imagick - Provides a wrapper to the ImageMagick library
php-libsodium - PHP wrapper for the Sodium cryptographic library
php-mailparse - Email message manipulation for PHP
php-memcache - memcache extension module for PHP
php-memcached - memcached extension module for PHP, uses libmemcached
php-mongodb - MongoDB driver for PHP
php-msgpack - PHP extension for interfacing with MessagePack
php-oauth - OAuth 1.0 consumer and provider extension
php-http - PECL HTTP module for PHP Extended HTTP Support
php-pinba - Pinba module for PHP
php-propro - propro module for PHP
php-radius - radius client library for PHP
php-raphf - raphf module for PHP
php-redis - PHP extension for interfacing with Redis
php-rrd - PHP bindings to rrd tool system
php-smbclient - PHP wrapper for libsmbclient
php-solr - PHP extension for communicating with Apache Solr server
php-ssh2 - Bindings for the libssh2 library
php-stomp - Streaming Text Oriented Messaging Protocol (STOMP) client module for PHP
php-uploadprogress - file upload progress tracking extension for PHP
php-uuid - PHP UUID extension
php-yac - YAC (Yet Another Cache) for PHP
php-yaml - YAML-1.1 parser and emitter for PHP
php-zmq - ZeroMQ messaging bindings for PHP
libapache2-mod-php7.0 - server-side, HTML-embedded scripting language (Apache 2 module)
libphp7.0-embed - HTML-embedded scripting language (Embedded SAPI library)
php7.0 - server-side, HTML-embedded scripting language (metapackage)
php7.0-bcmath - Bcmath module for PHP
php7.0-bz2 - bzip2 module for PHP
php7.0-cgi - server-side, HTML-embedded scripting language (CGI binary)
php7.0-cli - command-line interpreter for the PHP scripting language
php7.0-common - documentation, examples and common module for PHP
php7.0-curl - CURL module for PHP
php7.0-dba - DBA module for PHP
php7.0-dev - Files for PHP7.0 module development
php7.0-enchant - Enchant module for PHP
php7.0-fpm - server-side, HTML-embedded scripting language (FPM-CGI binary)
php7.0-gd - GD module for PHP
php7.0-gmp - GMP module for PHP
php7.0-imap - IMAP module for PHP
php7.0-interbase - Interbase module for PHP
php7.0-intl - Internationalisation module for PHP
php7.0-json - JSON module for PHP
php7.0-ldap - LDAP module for PHP
php7.0-mbstring - MBSTRING module for PHP
php7.0-mcrypt - libmcrypt module for PHP
php7.0-mysql - MySQL module for PHP
php7.0-odbc - ODBC module for PHP
php7.0-opcache - Zend OpCache module for PHP
php7.0-pgsql - PostgreSQL module for PHP
php7.0-phpdbg - server-side, HTML-embedded scripting language (PHPDBG binary)
php7.0-pspell - pspell module for PHP
php7.0-readline - readline module for PHP
php7.0-recode - recode module for PHP
php7.0-snmp - SNMP module for PHP
php7.0-soap - SOAP module for PHP
php7.0-sqlite3 - SQLite3 module for PHP
php7.0-sybase - Sybase module for PHP
php7.0-tidy - tidy module for PHP
php7.0-xml - DOM, SimpleXML, WDDX, XML, and XSL module for PHP
php7.0-xmlrpc - XMLRPC-EPI module for PHP
php7.0-xsl - XSL module for PHP (dummy)
php7.0-zip - Zip module for PHP
php-remctl - PECL module for Kerberos-authenticated command execution
php-tideways - Tideways PHP Profiler Extension
php-xdebug - Xdebug Module for PHP

Now you can choose and install php7.0. For instant:
# apt-get install php7.0 php7.0-gd php7.0-pgsql php7.0-cli php7.0-common php7.0-mcrypt php7.0-mysql php7.0-curl php7.0-json php7.0-cgi php7.0-xml php7.0-xmlrpc php7.0-bz2 php7.0-zip libapache2-mod-php7.0

Note:

1. Configuration folder will change from /etc/php5 to /etc/php/7.0/.
2. File php.ini will located in /etc/php/7.0/apache2/php.ini.