4     Functions

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Functions, both ones built into PHP and ones you define yourself, make coding much easier - they take away lots of hard work because you can reuse other people's code, and they allow you to keep your scripts shorter and easier to maintain. As PHP 5 includes more than 2,500 functions, you might assume it's a very easy language indeed, but the truth is that each function needs to be used in different ways and so needs to be learnt individually. In this chapter you will learn your first PHP functions, with the most helpful and easy first.

Rather than writing pieces of code time after time whenever you want to execute the same functionality, PHP allows you to encapsulate code into a named function that you can call from elsewhere in your script.

PHP comes with hundreds of predefined functions that perform all manner of tasks from reading files and manipulating strings up to querying databases and connecting to an IRC server. If you find something is missing, you can add your own functions on a script by script basis, and these are called user functions .

In this section we will be covering a variety of the most important basic functions in PHP - more specialised functions can be found spread throughout the book under various sections, and should be looked up using the index.

Topics covered in this chapter are:

• Working with date and time

• Mathematical functions

• String manipulation

• Creating data hashes

• Regular expressions

• Extension handling

• Writing your own functions

• Recursive, variable, and callback functions

Chapter contents

4.1. Functions overview
4.2. How to read function prototypes
4.3. Working with variables: isset(), empty(), and unset()
4.4. Controlling script execution: exit(), eval(), and die()
4.5. Working with Date and Time

4.5.1. Reading the current time: time() and microtime()
4.5.2. Converting from a string: strtotime()
4.5.3. Converting to a string: date()
4.5.4. Converting from components: mktime()

4.6. Mathematics

4.6.1. Rounding: ceil(), floor(), and round()
4.6.2. Randomisation: rand(), mt_rand(), getrandmax(), mt_getrandmax(), srand(), and mt_srand()
4.6.3. Trigonometrical conversion: sin(), cos(), tan(), asin(), acos(), and atan(), deg2rad(), rad2deg()
4.6.4. Other mathematical conversion functions: abs(), sqrt(), pow, and hypot()
4.6.5. Base conversion: bindec(), decbin(), dechex(), decoct(), hexdec(), octdec(), and base_convert()
4.6.6. Mathematical constants

4.7. Playing with strings

4.7.1. Reading from part of a string: substr()
4.7.2. Replacing parts of a string: str_replace() and str_ireplace()
4.7.3. Converting to and from ASCII: chr() and ord()
4.7.4. Measuring strings: strlen(), count_chars(), and str_word_count()
4.7.5. Finding a string within a string: strpos() and stripos()
4.7.6. Returning the first occurrence of a string: strstr() and stristr()
4.7.7. Trimming whitespace: trim(), ltrim(), and rtrim()
4.7.8. Wrapping your lines: wordwrap()
4.7.9. Changing string case: strtoupper(), strtolower(), ucfirst(), and ucwords()
4.7.10. Making a secure data hash: sha1()
4.7.11. Alternative data hashing: md5()
4.7.12. Automatically escaping strings: addslashes() and stripslashes()
4.7.13. Pretty-printing numbers: number_format()
4.7.14. Removing HTML from a string: strip_tags()
4.7.15. Comparing strings: strcmp() and strcasecmp()
4.7.16. Padding out a string: str_pad()
4.7.17. Complex string printing: printf()
4.7.18. Parsing a string into variables: parse_str()

4.8. Regular expressions

4.8.1. Basic regexps with preg_match() and preg_match_all()
4.8.2. Novice regexps
4.8.3. Advanced regexps
4.8.4. Guru regexps
4.8.5. Regular expression replacements: preg_replace()
4.8.6. Regular expression syntax examples
4.8.7. The regular expressions coach

4.9. Checking whether a function is available: function_exists()
4.10. Extension functions: get_loaded_extensions(), get_extension_funcs(), dl(), and extension_loaded()
4.11. Pausing script execution: sleep() and usleep()
4.12. Executing external programs: exec(), passthru(), and virtual()
4.13. Connection-related functions: ignore_user_abort(), register_shutdown_function(), and connection_status()
4.14. Altering the execution environment: ini_get(), ini_set(), and set_time_limit()
4.15. User functions

4.15.1. Return values
4.15.2. Parameters
4.15.3. Passing by reference
4.15.4. Returning by reference
4.15.5. Default parameters
4.15.6. Variable parameter counts: func_num_args(), func_get_arg(), and func_get_args()

4.16. Variable scope in functions
4.17. Overriding scope with the GLOBALS array
4.18. Recursive functions
4.19. Variable functions: is_callable(), call_user_func() and call_user_func_array()
4.20. Callback functions
4.21. The declare() function and ticks: declare(), register_tick_function(), and unregister_tick_function()
4.22. Handling non-English characters
4.23. Undocumented functions
4.24. Summary
4.25. Exercises
4.26. Further reading
4.27. Next chapter