Allow only [a-z][A-Z][0-9] in string using PHP

This:

preg_replace("[a-zA-Z0-9_-]", "", $var);

wouldn't even replace those characters, except if the input string is exactly the pattern. By using [] as delimiters, they have not the same effect as their would in the expression itself. You could change your delimiter (e.g.: /), or add some more brackets in the pattern:

preg_replace("/[a-zA-Z0-9_-]/", "", $var);    // this works
preg_replace("[[a-zA-Z0-9_-]]", "", $var);    // this too

Now, to negate a pattern in [], you use ^ at the beginning:

preg_replace("/[^a-zA-Z0-9_-]/", "", $var);

You could also have used the insensitive modifier i to match both lowercase (a-z) and uppercase (A-Z):

preg_replace("/[^a-z0-9_-]/i", "", $var);   // same as above

if (preg_match ('/[a-zA-Z0-9 ]/', $getname)) {