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PHP Bible
Chapter 16: Choosing a Database for PHP
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PHP Bible, 2nd Edition
1
 Wiley and the book authors, 2002
Summary
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What is a database?
Why a database?
Choosing a database
Advanced features
PHP-supported databases
Database abstraction (or not)
Our focus: MySQL and Oracle
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What is a database?
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Perhaps the single greatest advantage of PHP over similar
products is the unsurpassed choice and ease of database
connectivity it offers.
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PHP supports native connections to a number of the most popular
databases, open source and commercial alike.
Almost any database that will open its API to the public seems to
be included.
For any unsupported databases, there’s generic ODBC support
A database is a separate application that stores a collection of
data
Each database has one or more distinct APIs for creating,
accessing, managing, searching, and replicating the data it holds
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Why a database?
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If you’re going to the trouble to use PHP at all, you’re likely to need a
database sooner or later. Even for something small, like a personal Weblog,
you want to think hard about the advantages of using a database instead of
static pages or included text files
Maintainability and scalability
 Having PHP assemble your pages on-the-fly from a template and a
database is an addictive experience. Once you enjoy it, you’ll never go
back to managing a static HTML site of any size. For the effort of
programming one page, you can produce an infinite number of uniform
pages. Change one and you’ve changed them all.
Portability
 Because a database is an application rather than a part of the OS, you can
easily transfer its structure and contents from one machine to another or
even from one platform to another (we moved our MySQL database
from Windows NT to FreeBSD with very little effort)
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Why a database (cont.)
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Searching
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Although it’s possible to search multiple text files for strings
(especially on Unix platforms) it’s not something most Web
developers will want to do all the time. After you search a few
hundred files, the task becomes slow and hard to manage.
Databases exist to make searching easy. With a single command,
you can find anything from one ID number to a large text block,
to a JPEG image.
In some cases, information attains value only when you put it into
a searchable database (e.g. a long list of movies with actors
becomes much more valuable if you can search for all of the
movies that contain a particular actor, etc.)
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 Wiley and the book authors, 2002
Why a database (cont.)
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Security
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A database adds another layer of security if uses with its own
password or passwords
If you have a content site with a large number of visitors, a
smaller number of writers, and a handful of editors, you can easily
set database permission levels for each group so that visitors can
just look at the database content, writers can browse and change
only their own entries, and editors can browse/change/delete
anything on the site
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 Wiley and the book authors, 2002
Choosing a database
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Although databases (even relational ones) have been around for
a long time, they were quite expensive or limited in
functionality until very recently
You may not have a choice for the database used for your site
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You may have legacy databases to work with
Your OS may dictate what DB is used (or cannot be used)
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Choosing a database (types of databases)
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There are 3 main types of databases used with PHP
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Flat-file: (e.g. Gnu DBM and Berkeley DB) are used by or within
other programs such as e-mail servers. They provide the lightest
weight and fastest means of storing and searching for data like
username/password pairs or dated e-mail messages.
Relational: most common and widely used type of database.
Generally speaking, databases that speak fluent SQL can be
considered relational
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It’s important to note that some databases, although relational, are
not intended to be used in a multi-user environment (e.g. MS Access
& FileMaker Pro)
Objected-oriented/Object-relational: new and still developing
models of data access. Intended to work smoothly with objectoriented programming
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 Wiley and the book authors, 2002
Choosing a database (APIs)
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There are two generic standard APIs for database access: Open Database
Connectivity (ODBC) and Java Database Connectivity (JDBC). ODBC is
closely associated with Microsoft and JDBC is most closely associated with
Sun. Many other companies have incorporated these standards into their own
products.
 ODBC and JDBC support are more or less mutually exclusive. There are
bridges which allow applications to access an API of the other type but
they generally perform fairly slowly
 Some databases provide their own APIs which are much faster since they
have fewer layers to go through. They may also support ODBC or JDBC
but access should be through their own APIs if possible
Although ODBC and JDBC are slower than the native APIs, they do have
the advantage of being an open standard so PHP code written to the generic
APIs will mostly work with any ODBC or JDBC-compliant database so the
backend can be changed without modifying any of the code
 A better solution would be to write PHP-wrapper functions around the
database access functions which could then be swapped out to another
database-specific API while modifying only a minimal amount of PHP
code
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 Wiley and the book authors, 2002
Advanced features to look for
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GUI (Graphical User Interface)
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Databases vary enormously in their user interface tools. Choices
range from command-line interactions to massive Java-powered
development toolkits. You should look for the lightest interface
that meets your needs because a GUI can add substantially to
overhead costs
Some databases have available a web interface for administration
and maintenance which may be available from the manufacturer
or from a third-party. MySQL has several freely available
interfaces (notably PHPMyAdmin from www.phpmyadmin.org)
Subqueries
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A subquery or subselect is an embedded select statement like
SELECT * FROM table1 WHERE id IN (SELECT id FROM table2);
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This can be worked around if not directly supported by the
database, but support can save some time
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Advanced features to look for (cont.)
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SELECT INTO
 A handy feature allowing you to move data from one table to another
frequently
SELECT INTO table2(col2,col3,col7) lastname, firstname, state FROM table1
WHEE col5 = NULL;
INSERT INTO table2(col2,col3,col7) SELECT lastname, firstname, state
FROM table1 WHERE col5 = null
This should be avoided except under very special circumstances since it
will usually result in unnecessary data duplication
Complex joins
 A join is a way of searching for or retrieving information from across
multiple tables using shared values (foreign keys) to match up the tables
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SELECT * FROM table1,table2 WHERE table1.id=table2.table1_id
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Join operations can be specified as left or right, straight or cross, inner or
outer or self. Not all of these may be supported by all databases.
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Advanced features to look for (cont.)
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Threading and locking
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Threading and locking are very important for multiple-tier sites
and two-tier sites that have many contributors. They prevent two
database calls from conflicting with each other (e.g. preventing 2
people from purchasing the same seat at an event at the same
time)
Transactional databases
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This is a database design that seeks to maximize data integrity.
The transactional paradigm relies on commits and rollbacks.
Transactions that are concluded successfully will be committed to
the database. Those that are not successfully concluded will not be
saved (rolled back). This is especially important if you have
queries which update multiple tables at about the same time and
are dependant on each other (e.g. updating bank account data)
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 Wiley and the book authors, 2002
Advanced features to look for (cont.)
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Procedures and triggers
 Procedures are stored, precompiled queries or routines on the database
server. A common procedure would be one that selects out all the e-mail
addresses of customers who make purchases on a particular day.
 Triggers are procedures that occur when some event is registered by the
database (e.g. sending an account statement e-mail to customers every
Sunday). Triggers are rarely supported in databases
Indexes
 Indexes are a way to speed up searches of large data sets. Indexes
maintain the equivalent of card catalogs to the fields you specify
If you have millions of customers and you wanted to find the ones whose
last name were Smith, without the index the database would have to look at
each of the millions of records comparing the last name field to Smith but an
index on the last name field would allow a quick search on a small subset of
the database
 Indexes should not be used for every table or for every field in a table since
they slow down writes into the database and they don't speed up every type
of query. Indexes also can take a significant amount of disk space.
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 Wiley and the book authors, 2002
Advanced features to look for (cont.)
Foreign keys and integrity constraints
 The relational structure of a database is often implicit in the ways fields
of one table refer to row IDs of another, but your database won't
necessarily do anything helpful to make sure that the structure is
respected as changes are made (especially when deleting records). One
way the database can help is via cascading deletes – automatically
deleting records that depend on other records in other tables that have
been deleted
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Database replication
 As your data store expands and your business becomes more dependant
on its availability and performance, it is imperative to maintain the
database on more than one server. This will require a way to
automatically keep the different servers synched up. This will usually
involve a journaling system and often a master-slave relationship where
one server is designated the master into which all new data is added. A
journal keeps track of these changes in chronological order. The slaves
then read the master journal and make the same changes in themselves.
Select queries can then be made against the multiple slaves improving
performance and reliability. Another feature would be when if the master
were to fail, a slave could automatically be promoted to the master
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PHP Bible, 2nd Edition
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 Wiley and the book authors, 2002
Database abstraction (or not)
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Database abstraction refers to writing wrapper functions or classes instead of
using the bare PHP commands directly from your code. This methodology is
supported by many experts in application development and simultaneously
decried by many other experts.
 This issue is more apparent in PHP than other development languages
because of the huge numbers of databases and interfaces that are
supported
Argument for database abstraction mainly consists of the ability to swap
databases without having to change a lot of code
Argument against abstraction is that if you were to switch databases it may
dissuade you from taking advantage of some features available on the new
system
 Oracle will allow you to retrieve an entire data set from a query into an
array in a single operation. No other DB has that option.
 This could be overcome by writing a wrapper function which retrieves
the entire dataset and creates an array with all of the records returned
Other advantages with writing wrappers include a consistent error reporting
methodology, taking advantage of previous queries, and reducing the total
amount of code in your application
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MySQL and Oracle
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MySQL if quite likely the fastest, cheapest, simplest, and most
reliable database that also has most of the features you'd want
and comes in Unix and Windows implementations
There are certain situations in which you need the power of an
enterprise-ready transactional system like Oracle, however
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E.g. if your business requires of the vendor on-site support or the
ability to sue their pants off if something fails because of a
problem with the database
MySQL and Oracle pretty much represent the low and high
ends of the Web database server market. If you can write your
code to both of these, you should be able to write it to anything
else
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PHP Bible, 2nd Edition
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 Wiley and the book authors, 2002