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1. In php, functions are not case-sensitive, but variables are case-sensitive
2. Functions can define optional parameters, but if there are multiple optional parameters, the default values are consecutive from right to left , the middle cannot be skipped; when using this function, the parameters are from left to right
3. Put the frequently used functions in a file, and then import them with required().
4. The method to determine the data type is as follows:
isset(): //Whether the variable has been declared
empty(): //Whether the variable is empty
defined()://whether the constant has been defined define()
array_key_exists(mixed key, array search): //Check whether the given key name or index exists in the array
is_numeric ( mixed var ): // Detect whether the variable is a number or a numeric string
is_bool(): // Detect whether the variable is a Boolean
is_float(): // Detecting whether the variable is a floating point type has the same usage as is_double and is_real()
is_int(): // Detecting whether the variable is an integer is_integer() has the same usage
is_string(): // Check whether the variable is a string
is_object(): // Check whether the variable is an object
is_array(): // Check whether the variable is an array
is_null (): // Check whether the variable is NULL and whether the value is NULL. It is case sensitive
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