Encyclopedia of Computer Science and Technology

PL/I 373ing for many users. PHP can also be linked to user-interfacelibraries (such as GTK+ for Linux/UNIX) to create applicationsthat run on the client machine rather than the server.PHP has a basic set of data types plus one called“resource” that represents data processed by special functionsthat return images, text files, database records, andso on. Additionally, PHP5 provides full support for objects,including private and protected member variables, constructorsand destructors, and other features similar tothose found in C++ and other languages.There are numerous libraries of open-source objectsand functions that enable PHP scripts to perform commonInternet tasks, including accessing database servers (such asMySQL) as well as extensions to the language to handle popularWeb formats such as Adobe Flash animation. Programmershave access to a wide range of PHP resources throughPEAR (the PHP Extension and Application Repository).The combination of sophisticated features and easyinteractive scripting has made PHP the language of choicefor many Web developers, who use it as part of the group oftechnologies called LAMP, for Linux, Apache (Web server),MySQL (database), and PHP.Further ReadingAchour, Mehdi, et al. PHP Manual. Available online. URL: http://www.php.net/manual/en/. Accessed November 7, 2007.Lerdorf, Rasmus, Kevin Tatroe, and Peter MacIntyre. ProgrammingPHP. 2nd ed. Sebastapol, Calif.: O’Reilly Media, 2006.PEAR-PHP Extension and Applications Repository. Availableonline. URL: http://pear.php.net/. Accessed November 7,2007.PHP [official Web site]. Available online. URL: http://php.net/.Accessed November 7, 2007.Zandstra, Matt. PHP Objects, Patterns, and Practice. Berkeley, Calif.:Apress, 2004.PL/IBy the early 1960s, two programming languages were inwidespread use: FORTRAN for scientific and engineeringapplications and COBOL for business computing. However,applications were becoming larger and more complex, callingfor a wider variety of capabilities. For example, scientificprogrammers needed to provide data-processing andreporting capabilities as well as computation. Business programmers,in turn, increasingly needed to work with formulasand statistics and needed floating-point and othernumber formats.Language developers thus began to look toward a general-purposelanguage that could be equally at home withwords, numbers, and data files. Meanwhile, IBM was preparingto replace its previously separate scientific and businesscomputer systems with the versatile System/360. Theyand one of their user groups, SHARE, formed a joint committeeto develop a new language for this new machine.At first the designers thought in terms of extendingFORTRAN to provide better text and data-processing capabilities,so they designated the new language FORTRANVI. However, their focus soon changed to designing acompletely new language, which was known until 1965 asNPL (New Programming Language). Because this acronymalready stood for Britain’s National Physical Laboratory, thename of the language was changed to PL/I (ProgrammingLanguage I).Language FeaturesPL/I has been described as the “Swiss army knife of languages”because it provides so many features drawn fromdisparate sources. The basic block structure and controlstructures (see loop and branching statement) wereadapted from Algol, a relatively small language that hadbeen devised by computer scientists as a model for structuredprogramming (see Algol) and is also similar to Pascal(see Pascal). Blocks can be nested, and variables declaredwithin a block can be accessed only within that block andits nested blocks, unless declared explicitly otherwise.PL/I includes a particularly rich variety of data typesand can specify even the number of digits for numeric data.A PICTURE clause similar to that in COBOL can be used tospecify exact layout. However, the language takes a morepragmatic approach than Algol or Pascal; data need notbe declared and will be given default characteristics basedon context. Input/Output (I/O) is built into the languagerather than provided in an external library, and the flexibleoptions include character, streams of characters, blocks,and records with either sequential or random access.In general, PL/I provides more control over the lowleveloperation of the machine than Algol or even successorssuch as C. For example, there is an unusual amount ofcontrol over how variables are stored, ranging from STATIC(present throughout the life of the program) to AUTO-MATIC (allocated and deallocated as the containing blockis entered and exited) to CONTROLLED, where memorymust be explicitly allocated and freed. Pointers allow memorylocations to be manipulated directly. PL/I also providedmore elaborate facilities for handling exceptions (errors)arising from hardware condition, arithmetic, file-handling,or other conditions.Example ProgramThe following program executes a DO loop and counts fromone to the number of items specified. It then outputs thetotal of the numbers and their average.COUNTEM: PROCEDURE OPTIONS (MAIN);DECLARE (ITEMS, COUNTER, SUM, AVG) FIXED;ITEMS = 10;SUM = 0;DO COUNTER = 1 TO ITEMS;SUM = SUM + COUNTER;END;AVG = SUM / ITEMS;PUT SKIP LIST (“TOTAL OF ”);PUT ITEMS;PUT (“ITEMS IS ”);PUT TOTAL;PUT SKIP LIST (“THE AVERAGE IS: ”);PUT AVG;END COUNTEM;