PHP does not have a concept of variable type, I am assuming that your query is about the MySQL CHAR/VARCHAR variable.
CHAR is a fixed length data type. CHAR(100) will take 100 characters of storage even if you enter less than 100 characters to that column.
VARCHAR is a variable length data type. VARCHAR(100) will take only the required storage for the actual number of characters entered to that column. For example, "QueryHome" will be stored as "QueryHome" in VARCHAR(100) column and will occupy 8 bytes in storage (loosely saying).