Variables, Misc

Command-line arguments

$argv  // $argv[0]: script name; $argv[1]: first parameter, ...

Scalars

• Booleans, integers, floating point numbers, strings.

NULL

• To specify a literal NULL value, use the keyword NULL, which is case insensitive.
• Don't compare a variable to the keyword NULL (zero values cause it to misfire); use is_null() instead.

• A variable is NULL if

• It's been assigned the constant NULL.
• It hasn't been set to any value yet.
• It's been unset().

Variable scope

• By default, variables declared within a function are local
  
• Global variables must be declared global inside a function, using the keyword "global", if they are going to be used in that function.
• Global variables can also be accessed using the $GLOBALS associative array.

function Sum() {
    global $a, $b;
    ...
    $GLOBALS["c"] = $a;
    $x = $GLOBALS["c"];
}

Variable variables

• The name of variable variables can be set and used dynamically.
• A variable variable takes the value of a variable and treats that as the name of a variable.
• The indirectly-named variable, when accessed, will automatically be created if it does not already exist.

$var = "hello";
$$var = "world";      // Equivalent to: $hello = "world";

echo "$var ${$var}";  // "hello world"
echo "$a $hello";     // "hello world"

${$a[1]}   // Use $a[1] as an indirectly-named variable
${$a}[1]   // Use $$a as an array variable variable; access the [1] index

Variable parsing

• Used for double-quote strings and heredoc text.
• Variables can be placed inside curly braces, to ensure that they're expanded correctly.
• The $ character can be either inside or outside of the curly braces.
• Use "{$" or "\{$" to get a literal "{$".

$apple = "apple";
echo "$apple ${apple} {$apple}";
echo "${apple}s {$apple}s";
echo "{$fruits['banana']}";
echo "$rectangle->width {$rectangle->width}";