php - variable doesn't print the expected value? -

here code. database , tables created successfully, thing when try echo debug messages, way contains status , error messages acquired while running methods , queries. when try echo printing 00. not sure, think problem may using static (but want static way) , tried code in in different ways on own , after searching online different keywords, found lot didn't much.now, thing don't know search for, wonder searching wrong keywords? thought of posting here.

<?php //creates required databases & tables  class database {     public static $debug_message="";     //root     private static $host="localhost";     private static $root="root";     private static $root_pass="";     //c db     private static $user="c_user";     private static $pass="c_pass";     private static $db="c";     //tables     static $student="student";     static $lecturer="lecturer";     //field sizes     const small_text=25;     const medium_text=50;     const big_text=100;     const large_text=225;       public static function createdatabase()     {         self::$debug_message+="creating database".self::$db." user ".self::$user." password ".self::$pass;         try{             $dbh=new pdo("mysql:host=".self::$host,self::$root,self::$root_pass);             $dbh->exec("create database `".self::$db."`;                         create user '".self::$user."'@'localhost' identified '".self::$pass."';                         grant on `".self::$db."`.* '".self::$user."'@'localhost';                         flush privileges;")             or die(print_r($dbh->errorinfo(),true));         }catch(pdoexception $e){             self::$debug_message+=$e->getmessage();         }       }     public static function createtables()     {         self::createstudenttable();         self::createlecturertable();         //echo debugging         echo self::$debug_message;         return self::$debug_message;     }     static function createstudenttable()     {         self::$debug_message+="creating student table <br/>";         try {              $dbh=new pdo("mysql:host=".self::$host.";dbname=".self::$db,self::$root,self::$root_pass);              $dbh->setattribute( pdo::attr_errmode, pdo::errmode_exception );              $sql ="create table ".self::$student."(              student_id int( 11 ) auto_increment primary key,              firstname varchar(".self::medium_text."),              lastname varchar(".self::medium_text."),              nationality  varchar(".self::small_text."),              enrollment_status  int(1) default 0 not null);" ;              $dbh->exec($sql);         }catch(pdoexception $e){             self::$debug_message+=$e->getmessage();          }     }     static function createcoursetable()     {         self::$debug_message+="creating course table <br/> ";         try {             $dbh=new pdo("mysql:host=".self::$host.";dbname=".self::$db,self::$user,self::$pass);              $dbh->setattribute( pdo::attr_errmode, pdo::errmode_exception );              $sql ="create table ".self::$course."(              course_id int( 11 ) auto_increment primary key,              course_name varchar(".self::big_text."),              course_code varchar(".self::small_text."),              mqa_level int(2));" ;              $dbh->exec($sql);         }catch(pdoexception $e){             self::$debug_message+=$e->getmessage();         }     }     static function createlecturertable()     {         self::$debug_message+="creating lecturer table <br/>";         try {              $dbh=new pdo("mysql:host=".self::$host.";dbname=".self::$db,self::$user,self::$pass);              $dbh->setattribute( pdo::attr_errmode, pdo::errmode_exception );              $sql ="create table ".self::$lecturer."(              lecturer_id int( 11 ) auto_increment primary key,              first_name varchar(".self::medium_text."),              last_name varchar(".self::medium_text."),              nationality  varchar(".self::small_text."),              enrollment_status  int(1) default 0 not null);" ;              $dbh->exec($sql);         }catch(pdoexception $e){             self::$debug_message+=$e->getmessage();         }     }  }  //tried 2 ways debugging nothing works  //static way database::createdatabase(); echo database::createtables(); //prints 00  //instance $db=new database(); $db->createdatabase(); echo $db->createtables(); //prints 00 

result

0000 

but expect print following

creating database c user c_user  password c_pass //if error while creating database, error message here creating student table //if error while creating student table, error message here creating lecturer table ////if error while creating lecturer table, error message here 

when directly use strings inside , echo static functions strings prints when assign variable , tries echo main function of instance doesn't work.

any appreciated. in advance :)

the problem has nothing use of static @ all. rather, problem concatenate strings in php, use . , not +.

instead of

self::$debug_message+="creating student table <br/>"; 

you should have

self::$debug_message.="creating student table <br/>"; 

an output of 0 happens when try add 2 strings together.