SQL Server has no exact equivalent of the mysqldump utility.
As Katherine said, SQL Server's default backup format is a binary file. It serves the same basic purpose as a dump file - you use it to restore a database. But you can't read it in a text editor - you need special tools to interpret it.
If you just want a backup of the whole database or a single table, there are a couple of utilities you can invoke using PHP's exec function.
To backup a single table, use the bcp utlity.
bcp dbo.$dbtable out $name.bcp -U $dbuser -P $dbpass -S $dbhost -d $dbname
To backup the whole database, use the sqlcmd utility and the BACKUP DATABASE command.
sqlcmd -S $dbhost -d $dbname -U $dbuser -P $dbpass -Q "BACKUP DATABASE $dbname TO FILE '$name.bak';"
If you need something more like mysqldump, you could use a PowerShell script or a third-party tool.
With PowerShell SMO (Server Management Objects), you can use the Scripter class and the ScriptingOptions class to script any object.
While researching the answer I found a third-party tool called sqlserverdump, "a command line utility inspired by mysqldump". I've not tried it yet, but it might be just what you need.