variable
1. Variables must start with $, followed by the variable name.
2. Unlike JS, it does not support 'define only without assignment'. Assignment is to make one variable name use another value;
3. Two methods of variables:
1. isset(): Judging whether the variable name exists, returns 1-true if it exists, and returns 0-false if it does not exist. If the value of a variable is Null, it does not meet the variable definition rules, and it returns false.
2.unset(): disconnect the "reference relationship" between the variable name and the data
4. Pass-by-value (pass-by-value/pass-by-reference)
Pass-by-value: copy a data value (data) and assign it to another variable. After the assignment, the two are independent of each other
Pass-by-reference: Copy and pass the reference relationship at the same time. Key symbol & Example: $a=1; $b=&$a
5. Variable variables
refers to the name of a variable, which is another variable example:
$v1 = "abc"; // this is a string variable whose content is the string " abc "
$abc = 10; // This is a normal variable whose content is the number 10
echo $$v1; // At this time, it is the so-called "variable variable"
understanding of variable variables
1 , whenever the $ sign appears, it may be understood as a variable
2 , where the first " $ " appears, then php will interpret it as a variable, and the variable name is $v1;
3 , we know that the value of $v1 is "abc",
4 , then, the variable name after the first " $ " symbol is obtained as " abc "
5 , that is: what echo tries to output is the variable $abc , which is naturally the number 10
In fact, in theory, mutable variables can be nested more, such as: $$$def;