Even though sed is commonly used for these types of tasks, it isn't actually designed for them—its very name is stream editor.
The tool that is designed for non-interactive file editing is ex. (vi is the "visual editor" form of ex.) Particularly when you want to edit files in place, ex is a far superior tool to sed.
In this case the commands used are almost identical to the sed command given earlier, but they don't have to be. The following is POSIX compliant (unlike sed -i).
for file in *.php; do ex -sc '%s/[ăâ]/a/ge | %s/ș/s/ge | %s/ț/t/ge | %s/î/i/ge | x' "$file" ; done
Explanation:
-s starts "silent mode" in preparation for batch processing. -c specifies the command(s) to be executed.
% means to apply the following command to every line in the file. The s/// commands are fairly self-explanatory; the e flag at the end means that any errors (due to the pattern not being found) are suppressed and file processing will continue.
| is a command separator (not a pipe).
x tells ex to write any changes to the file (but only if there were changes) and exit.
If you want in-place file editing, ex is the tool of choice. If you want to preview the changes before you make them, I'd recommend using tr as @gardenhead suggests.
(Of course, if you're using a proper version control system such as git, you could make the changes in place using ex and compare the files to the old version by running git diff.)
echo ă â î ș ț | iconv -t us//TRANSLITwhich outputsa ^a ^i s ton OS/X (anda a i s ton GNU).