Question

How to define data type in PHP?

Answer 1

There is no need to define data type of variable.
PHP provides a facility which allows to program to perform operations in the way of how variables assigned.
If you wrote
$a=5;        // it will acts as int
if you wrote
$a='5';      // it will acts as string

Comments

So it means it defines variable type automatically in context?

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Yes. There is no need any data type to any variable.
You can assign any type of value afterwards you have declared. (still no need to declare a variable  you can directly use any variable w/o declaration).