Issue with charset and data - php

Question

I try to explain the whole problem with my poor english:
I use to save data from my application (encoded on utf8) to database using the default connection of PHP (latin1) to the tables of my DB with latin1 as charset.
That wasn't a big problem : for example the string Magnüs was stored as Magnüs, and when I recovered the data I saw correctly the string Magnüs (because the default connection, latin1).
Now, I change the connection, using the correct charset, with mysql_query("SET NAMES 'utf8'", $mydb), and I've also changed the charset of my tables's fields, so the value now is correctly store as Magnüs on DB; Then I still seeing Magnüs when I retrieve the data and I print on my Web Application.
Of course, unfortunatly, some old values now are badly printed (Magnüs is printed as Magnüs).
What I'd like to do is "to convert" these old values with the real encoding.
ALTER TABLE <table_name> CONVERT TO CHARACTER SET utf8; will convert only the field type, not the data.
So, a solution (discovered on internet) should be this:
ALTER TABLE table CHANGE field field BLOB;
ALTER TABLE table CHANGE field field VARCHAR(255) CHARACTER SET utf8;
But these old string won't change on database, so neither in the Web Application when I print them.
Why? And what can I do?

Answer 1

Make sure that your forms are sending UTF-8 encoded text, and that the text in your table is also UTF-8 encoded.
According to the MySQL reference, the last two ALTER you mentioned do not change the column contents encoding, its more like a "reinterpretation" of the contents.
Warning
The CONVERT TO operation converts column values between the character sets. This is not what you want if you have a column in one character set (like latin1) but the stored values actually use some other, incompatible character set (like utf8). In this case, you have to do the following for each such column:
ALTER TABLE t1 CHANGE c1 c1 BLOB;
ALTER TABLE t1 CHANGE c1 c1 TEXT CHARACTER SET utf8;
The reason this works is that there is no conversion when you convert to or from BLOB columns.