I am not familiar with Phing, but it cannot execute the command in the shell, so the redirection operator will not work (it can process the operator as an argument itself mysql).
< ${sql.file} -e 'source ${sql.file}', :
<exec command="mysql -u${mysql.username} -p${mysql.password} -h ${mysql.server} ${mysql.database} -e 'source ${sql.file}'" />
MySQL.