PHP: Built-In Functions

Derek Bridge

Department of Computer Science,
University College Cork

PHP: Built-In Functions

Aims:

• to learn how to use a few of PHP's built-in functions
• to learn how to make sense of the PHP manual

Motivating example

• We wrote a program to convert RGB values using percentages to RGB values using decimal numbers between 0 and 255 inc.
• But the program isn't right
• Class exercise: What's wrong with it?

Functions

• In PHP, a function is a named block of PHP that performs some task
  • it takes in zero, one or more values (its actual parameters)
  • it may return a value
• PHP has many built-in functions
• E.g. sqrt returns the positive square root of its actual parameter
  • sqrt(9) returns 3

The hypotenuse of a right-angled triangle

• Suppose right_triangle.html contains this form:

  <form action="hypotenuse.php" method="get">
      <input type="text" name="c1" />
      <input type="text" name="c2" />
      <input type="submit" />
  </form>

• Write hypotenuse.php, which calculates the length of the hypotenuse of the right triangle, where the length of the sides adjacent to the right-angle are input into the form above
• Hypotenuse by Pythagoras' theorem

More functions

• abs($x) returns the absolute value of $x
  • E.g. abs(3) returns 3
  • E.g. abs(-3) returns 3
• pow($x, $y) returns $x$y
  • E.g. pow(3, 4) returns 81
  • E.g. pow(1000, 1/3) returns 10
• Lots more: sin, asin, asinh,… even hypot!

A key skill: reading the documentation

• In PHP documentation, functions are described by giving their prototype and some sentences of explanation
• E.g. the PHP manual: http://www.php.net/manual/en/
  • the entry in the PHP manual for the floor function
  • the entry in the PHP manual for the ceil function
• The prototype specifies
  • the return type
  • the name of the function
  • the type of each parameter
• Class exercise: What are the differences between using (int) and floor?

A key skill: reading the documentation

• Some functions may have one or more optional actual parameters
• E.g.
  • the entry in the PHP manual for the round function
• The square brackets indicate an optional parameter

RGB conversions, again

• Modify rgb_convert.php, so it converts the percentages to the nearest decimal integer between 0 and 255 inc.