PHP Variables

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PHP Variables
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What is a variable?

• A variable is something that can be changed a
  thing that can take on any value from a possible
  set of values.
• For example, the number of hours someone works in
  a given week is a variable.
• In math, a symbol is used in place of the
  variable to express a relationship between some
  quantity and the variable
• For example, pay is ones pay rate times the
  number of hours worked
• Pay PayRate NumberHours

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What is a variable? (Cont.)

• So a variable is something that stands in for a
  quantity that might vary.
• In programming, a variable can be thought of a
  referring to a memory location or set of memory
  locations. The value of the variable at any
  given time is then the contents of that memory
  location
• As we said a variable can take on values from a
  set of possible values, the set of values is
  determined in part by the variables type.

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Types

• Unlike many other programming languages, PHP
  variables do not have to be declared. In such
  a case, a variables types tends to come from its
  context.
• Standard types includes numbers (both integers
  and so-called floating point numbers that can
  have fraction parts.
• Another standard type is a string,which can used
  for names of things or just about any text.

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PHP variable rules

• In PHP a variable starts with a dollar sign.
• After that must come a letter or an underscore
  (no numbers).
• After that can come letters or numbers.
• There can be no spaces in the name.

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Assignment

• A variable is said to be assigned a value in an
  assignment statement. E.g.
• NumberHours 45
• WorkerFirstName Pete
• The assignment statement should not be thought of
  as expressing mathematical equality rather it a
  set of instructions to evaluate the expression on
  the right hand side and give the variable on the
  left hand side the value that results. Thus
• Counter Counter 1
• Makes sense as an assignment statement even
  though it makes no sense as mathematics.

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Turning DateDisplay into a program with a
variable.
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Result of DateDisplay as variable.
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Incorporating variables within strings.

• print "The date is dateString."
• The above code also shows how variables are used
  in conjunction with strings in PHP.
• Within the string (set of characters within
  quotes) that will appear literally (as is), the
  variable does not appear as is, rather its value
  is substituted into the string.

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If you dont want the variable value substituted
in, then use single quotes.
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Result of single quoted version of DateDisplay.
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That variable doesnt vary much.

• Because these are PHP variables, all of their
  variation takes place on the server side (before
  the PHP preprocessed page is sent to the client).
  
• PHP variables will ultimately give us interaction
  with the viewer of the page but always through a
  process in which information is sent back to the
  server.

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The bells, bells, bells example.

• Before consider variables determined by
  interaction with a user, let us consider one
  example in which the variable does vary.
• In Edgar Allen Poes poem The Bells, the word
  bells appears over and over again. Lets
  substitute in an image of bells in place of the
  word bells.
• So far substitution (a lot of it) but not
  variation.

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The bells, bells, bells example. (Cont.)

• The first part of the poem talks about silver
  bells, the second about gold bells, and so on.
• So in this example we will vary the image and the
  font color for each of the different parts.

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Assigning the variables
Writing some HTML
A chunk of the poem, with the word bells replaced
with the variable bells. Note that the
quotation can be spread out over many lines.
(That would ne a no-no in C.)
Re-assigning the variables
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Where we left off.
More of the same.
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Silver bells turning into Golden bells as a
result of our variable being reassigned.
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Constants

• If you really want a quantity that is going to
  take on a value and never change then what you
  want is called a constant.
• Constants do not start with dollar signs.
• By convention, constant names are in all capital
  letters.

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Pi page (code)
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Pi page (in browser)
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PHPs Global Constants
The ltpregt tag tells HTML to respect the white
space of the text between the tags.
The function print_r is for printing arrays
GLOBALS is not one constant but a set of constants
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Some of these constants tell one information
about the PHP server in this case the alpha.
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