Variables, which always start with $ in PHP, can hold all sorts of data for the life of the script
If you want to set a value that will not be changed, use a constant. From a speed perspective, there is little difference, however it makes clear in your code that the value cannot be changed by other programmers.
PHP has a large selection of operators to allow you to manipulate your variables in a simple way. Each operator has precedence and associativity, but you should not rely on these.
Some operators, such as ++, vary their effect depending on where they come in relation to a variable.
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