check if the given input has the expected type

how do i enforce constraints on input types
for example correct input i.e if i need to enter text not numbers
i should be given an error message when text is entered.

how do i guard against empty fields.

i also have a problem with entering data with space inbetween.

for example if i have the field name and i enter "john doe"

when i refer to the variable $name in an sql query it doesn't work
because only "john" is seen.

how can i solve this.

Sounds like you need some regular expressions. Do a search for regex, grep and ereg. There is a lot of information out there about text parsing. I believe there is even a site that has "recipes" for regular expressions, but I can't remember where it is right now.

As far as checking for empty fields, that depends on your programming language. Most have a construct like (isempty($x)) or (length($x) = 0) that you can use.