# <?php  
# /* 
# Credits: http://www.bitrepository.com/ 
# */  
#   
# // default chars values are defined  
# function validate_password($password, $min_char = 4, $max_char = 20)  
# {  
# // Remove whitespaces from the beginning and end of a string  
# $password = trim($password);   
#   
# // Accept only letters, numbers and underscore  
# $eregi = eregi_replace('([a-zA-Z0-9_]{'.$min_char.','.$max_char.'})',  
# '', $password);  
#   
# if(empty($eregi))  
#     {  
#     return true;  
#     }  
#     else  
#     {  
#     return false;  
#     }  
# }  
#   
# $password = 'my_password_123';  
#   
# /* 
# First parameter: Password 
# Second parameter: Minimum chars 
# Third parameter: Maximum chars 
# */  
#   
# $validator = validate_password($password, 4, 20);  
#   
# if($validator)  
# {  
# // Our current example will return true  
# echo 'The password is valid.';  
# }  
# else  
# {  
# /* if our password value is for instance: my_@#%_password 
# our function will return false */  
# echo 'The password appears to be invalid.  
# It should contain only letters, numbers and underscore.';  
# }  
# ?>

http://www.bitrepository.com/web-programming/php/validate-input-password.html

This is a useful function if you need to validate a password (input). If you have a form and you need to check if the username, which is registering, entered a valid password (without illegal characters) then this would help you to do it. Only letters, numbers and underscores are accepted.

Validate (input) password

PHP