Description:
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Currently the documentation for the function idn_to_ascii describes the first parameter as following:
>> Domain to convert. In PHP 5 must be UTF-8 encoded.

The truth is that this value do not have to be encoded in UTF-8. The encoding of this string depends on the setting of idn.default_charset

This setting by default is set to ISO-8891-1 and therefore a string containing a utf8-encoded character that's invalid in ISO-8891-1 will cause the parser to crash (ending up in an "segmentation fault" message).

Try the test-script once with the ini-value idn.default_charset of "ISO-8891-1" and once of "utf-8". Using the default-value (iso-8891-1) the script will break and using "utf-8" instead it will work as expected.

Test script:
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<?php
var_dump( idn_to_ascii('ә') );

echo "123";

Expected result:
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PHP Notice:  IDN_IDNA_TO_ASCII: Could not convert from IDNA to ASCII in /var/www/index.php on line 1

123

Actual result:
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PHP Notice:  IDN_IDNA_TO_ASCII: Could not convert from IDNA to ASCII in /var/www/index.php on line 1

Segmentation fault